How about “caught up in a whirlwind romance”?
I found this wording amusing.
My uncle has twice married after knowing a woman for a week. The first didn’t go very well, but the second lasted til the day she died.
Which, we’re going to assume, was quite some time after the marriage, right? RIGHT???
Heh. I knew a guy in college who said he and his girlfriend were “pre-engaged.” That term always amused me. They split up before freshman year was over.
Scuba Ben, I have no real advice except to say that if things are going so well, there’s no need to put a name to it; just enjoy. (But if you must, what’s wrong with just saying she’s your girlfriend and you’re her boyfriend?)
I don’t neccesarily agree. I know my mother wouldn’t.
My Mom and Dad met on a double date-she was with another guy and he was with another girl.
They were both attractive and in their late twenties with plenty of dating experience …keep in mind that this was in the early 1950’s.
But they hit it off really well and reached an “understanding” within a month. My Dad sent my Mom a card every day for 6 weeks or so ( he was an artist ) asking her to spend the rest of her life with him in various cute and creative ways, then they became engaged offically and married 3 months later.
My Mom, understandably, became a great believer in “love at first sight”
The marriage lasted and was basically happy until my dad passed 5 years ago.
As for the answer to your question, I would say that you and the young lady have an “understanding”.
Confirmation bias.
I’m just interested in finding out if the relationship lasted through the third week.
Seriously. It’s been, like, 10 days. Maybe the crazy kids eloped…
10 days? Maybe they’re on to their divorce.
Well I didn’t wanna be all negative-like…
She’s your gal and you’re her guy; enjoy!
As far as ‘When do you know for sure?’ well…it’s different for everyone. My relationships tend to last around 3 years, and even that hasn’t been long enough to convince me to marry. The joke is that one day someone will have to swoop me up while I’m still in the 'OMFG ECSTATIC!! stage or it’ll never happen. 
When it seems right, go for it; time frames don’t really matter except in the ‘Are you SURE he isn’t a manic-depressive psycho-killer?’ way. Or the ‘Are you SURE she isn’t a gold-diggin psycho-bitch?’ way. Either you can work problems out or you can’t; that part’s kind of important, and you need to HAVE some problems to find that out, which rarely occurs during the OMFG ECSTATIC stage, so yah. Talking about it’s good. Just wait until you’ve had a couple-few disagreements to see how you handle the downturns, and then go for broke if you’re both still standing afterwards.
Congrats 
I am really, really sorry but when I hear or read the word “soul mate”, I just can’t take it seriously. Besides that my friend, I think you really have to take your time and make sure you actually know this person (and vice versa). Best of luck.
Congratulations!
(I seem to remember you contributing advice to the OPs of a number of ‘why am I so unsuccessful in dating’ threads, so I’m glad you’ve found someone too!)