For the early birds, this will be a poll, please wait for the options
Seeing some old sit-coms, I am amazed how many married folk refer to their spouses parents as Mr./Mrs. lastname.
I am not married, but I don’t think I know anyone in my generation ~50 address that address their in-laws as Mr/Mrs. It is either by given name, Mom/Dad or nickname.
Edit, it is multichoice poll for those that might address their inlaws differently.
By their given names only.
Shortly after I’d joined the family I used to periodically hear my F-I-L refer to my M-I-L as “the big woman”, totally endearing, just meaning that she was often the decision maker. Well I screwed up and one time on the phone called her “Big Momma.” Not my greatest moment.
Oni no Husband’s extended family is rather shocked that I call them by their first names. Apparently I’m supposed to call them Mom/Dad. (It’s a cultural thing - except for my very white MIL, the extended family is almost entirely Filipino.) My MIL did suggest once that I call her “Mom”, but I never have.
I do sometimes call my MIL by a nickname, but I’m not sure if that’s considered to be better or worse.
This is a family where you don’t call your elder sibling or cousin by their unadorned first name, hence the scandal that I would do so with people who I should be having even more respect for!
Fortunately, because I’m a white girl they make allowances. Also, I have made it clear by my actions over the last 11 years that I love my inlaws and do respect them greatly, so it’s less of an issue.
My MIL & FIL have never made an issue over it, it’s just the aunties/uncles that used to have problems.
FIL given name, MIL her nickname, since no one uses her given name. To be fair, I knew both of them before I met/courted/fell in love with/married/knocked up their daughter.
First names - it’s how they were introduced to me, and it would be very weird for me to call them Mr and Mrs Lastname. Not as weird as Mum and Dad would be though - I already have a Mum and Dad; they’re his Mum and Dad, and his sister’s. I’m not his sister, I’m his fiancée: very different roles 
As deceased.
Typically by first name. I have 4 inlaws, my SO’s parents who have been divorced about 20 years and their current spouses. He calls his step-parents by their first names and so do I. Partially because his stepdad is only like 4-5 years older than me and calling him dad would be strange. I sometimes call his mom “mom” bu not usually.
Luckily, I’ve only got the one, and I use her first name. Our relationship is one where I would feel like a huge hypocrite calling her an endearment of any type. She would love it, though.
This. I can’t imagine referring to in-laws by anything other than their name, unless I had kids, in which case in front of the kids I would probably refer to them as ‘grandma/grandpa firstname’.
I’ve addressed my partner’s mother as mum once, in a greetings card, to please her. I occasionally address her as “Mother”, because she hates it and it makes us laugh. I usually call both of them by their first names. When I first met them I greeted them as Mr and Mrs Lastname, but they didn’t like it and put me straight right away.
When speaking to them, I call him by his first name, and her generally by “Miz Lastname” or just Ma’am. She’s an elegant southern lady, after all. They’d be fine with my calling her by her first name (that’s how my wife addresses my mother), but I’m just not comfortable.
I call them mom & dad. They’re awesome, and I’m very close with them.
My wife is more reserved and calls my parents by their first names.
45 years old, and Mrs. Lastname. She’s not my mother, and calling her by her first name would be disrespectful: that’s just the way I was taught by my grandma.
I call my husband’s biological mother by her first name. I call his father Dad or occasionally his name. And I call his step mother by her first name. Of course, with the Gothlette, there’s a lot more Grandma/Grandpa Firstname being tossed around.
I don’t like his bio mom and don’t feel she deserves to be called Mom. I have a bad relationship with my own father, so I enjoy having someone to call Dad. And the stepmom in law is a quiet, old fashioned lady who has never requested I call her Mom. Husband calls her Firstname so I follow suit.
It can get complicated, can’t it?
My wife’s parents divorced years ago, and have remarried other people. I call my MIL “Mom,” and my FIL by his first name. Her step parents I call by their first names as well.
First names but only if I have to.
I first met them when I was 14, so it was Mr & Mrs Lastname, of course.
Once we got engaged, FIL gave me a huge, welcoming grin and said “You’ll have get used to calling me Dad now”. Eyes turned to MIL, who pursed her lips and went “Hmmmph”
So, FIL has been “Dad” ever since, and I love him a lot. I avoided addressing MIL as anything for years and years (it’s fairly easy, especially when you don’t have much to say to each other!). I’ve occasionally used her first name, when talking to others, but never directly to her. The relationship has thawed over the years, but I wouldn’t call it warm yet.
I call my husband’s father and stepmom by their given names.
His mom and stepdad (who raised him) have asked me to use their first names, but I often refer to them as “Mama First-Name” and “Papa First-Name,” partly because my mother-in-law shares a first name with a close friend, and partly because it seems more respectful to use some kind of honorific. They are very dear people, and have welcomed me and their stepgrandchildren into the family wholeheartedly. I really love them!
Grandparents were a little more challenging. I called my recently-deceased grandmother-in-law “Dr. First Name,” because there was no way in hell I was going to address a 98-year-old woman by her first name, nor did I know her well enough to call her Grandmother. Similarly, I call my grandfather-in-law “Mr. Last Name.” He’s very much a Southern country gentleman, and 97 years old, and I’d never dream of calling him anything less formal. (He’s also quite deaf, so he doesn’t really know what I call him anyway, but I know.)
My husband calls my parents by their given names, and my grandmother “Mrs. Last Name.”
The poll results are showing me what I expected. Few people formally address their in-laws as Mr./Mrs. Surname.
At age 50, in a social setting, there is no one that I address as Mr. or Mrs.
I’m in my late 30s, but my brothers-in-law are 10-20 years older, and they call our FIL/MIL by “Mr./Mrs. Lastname.” I know my FIL doesn’t like it, either. First names doesn’t seem respectful - especially since they’re 20 years older than my parents - and I would never call my FIL “Dad.”