2 multi-year relationships, and a few 1ish year things. In my experience, yes sex does slow down.
In the 1ish year relationships, the sex didn't really slow down until things were pretty much done.
In the multi-year relationships my sexual frequency took a tumble in the 2.5 - 3.5 year range. In the early part of any relationship I was always a 1 and 1 guy. Jerk off once a day, and have sex once a day (less during her period). When things slowed down I was still jerking off once a day, but sex dropped off to maybe once or twice a week. In both cases we still loved each other, just sex became less of a priority. When I start dating somebody, and I think a lot of guys are like this, you pretty much have as much sex as you can like a starving man who found a free hotdog stand. Then you kind of settle in to a more normal routine. This is lots of fun, and there's lots of exploring, and you get really good at getting each other off. Some years go by and then it just kind of tapers down.
Personally, I wouldn't worry about it unless the sex stopped, or the sex isn't mutually satisfactory. In my case, it was also about this time when 'sex with anybody else' started becoming more of a pull. It's just that I'm the sort of person who's dumb enough to (try to, with some failure,) obey a vow of monogamy. At this point I actually avoid anything too serious, just because I'm having a decent amount of sex / fun without that, and because I don't want to make promises I'd have a hard time keeping.
When the 'taper down' really starts, and what it tapers down to, really depends on the people involved. I have a fairly recently married friend, who was only really having sex a couple of times a week when things were hot and heavy, and now they're down to once every couple of months. You can tell he sincerely loves his wife, and he's OK with that. I wouldn't be, but YMMV.