On my hardest FA, King Cobra at Rumney I think I spent about a month of weekends actually trying to climb it, maybe a little more. Most of my harder projects are FAs, so I spend a lot of time cleaning and bolting them, usually climbing sections over and over again while doing that, so it is hard to quantify actual climbing time. A lot of them I can't get back on regularly too.
Some projects take me years since once I have TRed a project, if I don't manage to lead it right away, I usually have found some other potential line that I am all excited about which takes my attention. Regarding other people's routes, Ward's Rocket Man at Rumney kept me working on and off for a several years. I managed to TR it a couple times, but was never on it consistently enough to lead it successfully; I kept having to relearn it. I don't mind working on a good route for a long time, but my attention is easily distracted by other new lines. At any crag I go to I am pretty much immediately looking at new route potential. My goal this year is not to allow myself to get distracted and clean up and finish at least 5 old projects before any new one, lol. I had to make a spreadsheet this winter to try to remember them all.