Me too, and sometimes it's incredibly frustrating. It usually takes me about 45 mins to 1 hr, but had occasions where it's taken getting on for 2 hours. My partner is usually fine with this and enjoys a good session (we only see each other a couple of times a month so it's not like it's daily), but often we give up and stop before I reach O. Very occasionally, it happens much quicker - 15 mins or so.
I never used to be like this. In my 20's I sometimes had the opposite problem, otherwise could usually pretty much cum on demand - when both of us were ready for it after about 20 minutes. Sometime in my 30's things started taking a lot longer. I'm definitely less sensitive down there than I used to be, which is normal for men to lose sensitivity as they age.
Other things I've noticed that affect how long I take to O, are extended foreplay before PIV - the longer that goes on for us (sometimes we have 30 mins to an hour of foreplay, cunnilingus etc, which is almost totally focussed on dp), the more the initial excitement has subsided, and the longer the sex lasts. Also, my dp likes piv to be hard and fast right from the start, which I suspect desensitises me somewhat. I prefer to start slow and build up pace. Usually my partner dictates the speed, but in certain positions I control it and keep things slower at the start.
Also, if it's not happening, a 5 minute break or a change position can sometimes quickly trigger an O. Rarely the same position that worked last time has the same effect next time though.