Sure, British series count. So Midsomer Murders, Misfits, and Doctor Who all fit the bill.
Wow. I hope her character has been given a promotion.
Tons. She’s an LT in charge of the squad. She still has to go out on investigations, though.
ETA: Since SVU is now the longest running show in prime time, I guess that means that Hargitay holds the record for playing a single character the longest.
Unless some of the actors who played a character across multiple shows beat her. How many seasons did Richard Belzer play John Munch when you add SVU and Homicide together? or Carroll O’Connor play Archie Bunker across two shows? Or Ed Asner play Lou Grant?
Coronation Street still features Ken Barlow as a regular character, who appeared in the debut of the show in 1960. Still played by the same actor too.
Isn’t The Simpsons the longest running scripted show in prime time, at least in the US? And AFAIK all its original main cast are still there.
That’s what I was going to say about Voltron. Though I think some of the characters might have had the same voice-actor for the American dub.
For the record for playing a single character the longest, it could be Peter Falk and Columbo. It was on from 1971 to 2003. But, there were a lot of gaps.
Skins in the UK changes its primary cast every two seasons. One character stayed four years, and came back in the final season, but there were no original members for two years. It’s about teens and the producers have them move on as the get too old for school.
I think the cast of The Simpsons has got him beat. The series has been on the air for 32 uninterrupted seasons, plus another couple years’ worth of sporadic appearances on The Tracey Ullman Show before that.
Frasier Crane (Chelsey Grammer) was on Cheers and Fraiser for a total of 20 seasons, so I think they’re tied right now.
Sam Waterson played Jack McCoy for a total of 24 years on a variety of shows. There are also several actors on British shows that have played the same character for considerably longer. Peter Sallis was in every episode of The Last of the Summer Wine for 31 seasons over 37 years.
I was going to say Sliders might be one, but Cleavant Derricks was there throughout, and also it depends on how strict your rules are as the previous full-timers reappeared in the final season, though only as guest stars.
That should read Kelsey Grammer, too late to edit.
Nope.
Okay, okay… how about Columbo may be the longest US, prime time, non-animated character.
But, we have strayed from the original question.
If you take out the US part, Bill Roach has played the same character in Coronation St since 1960. It’s a soap, but it’s primetime.
It’s become The Lt Benson Show. 
I can’t wait until Mariska Hargitay leaves, however I fear that when she does they will cancel the show. (I used to like her character until she became soooo into promoting her personal causes at the expense of storytelling.)
The Walking Dead had a full cast turnover, didn’t it? Sorry if this already mentioned, I failed to see or find it.
The original Being Human only had three main characters, but it found a replacement for each one in the span of five bite-sized British seasons. To my considerable surprise, I quite liked each of the new cast members.
A brief and non-OP-conforming mention to the cast of the world’s best cop show - Reno-911. A cliff-hanger ending to Season 2 saw them all sacked and imprisoned. The Season 3 opener had their characters replaced by competent versions of themselves. It was a single episode and soon enough things got back to normal, but it technically was a cast turnover in a sit-com.
Interestingly, like Robotech, Voltron was supposed to be made up of 3 Japanimation series. But when the “vehicle” team episodes flopped following the “lion” team, all that ever appeared of the “gladiator” team version was the toys produced by Matchbox (in the US).
And he’s played the recurring villain Sideshow Bob for most of the Simpsons 32 seasons. As has Joe Mantegna as Fat Tony, though technically he died and was replaced by his thin cousin Fit Tony, who then became fat and changed his name to Fat Tony. Same actor voices him though. Joe even once complained when Tony had a small bit part groaning or saying oof or something minor and rather than having him come do the part they just had a regular cast member do it.
Only the since departed Rick (and Shane and Lori and Carl in pre-zombie scenes) was in the first episode. But Darrell is still around from the first season. Morgan was in the first ep, then not on the show many years, then came back, then left again, and is now on Fear the WD.