My experience:
I had osteoarthritis in left hip but did not know it until 2012. Very gradual decline (I'd say over 5 or 6 years)
It started when I noticed very slight pain when shifting (manual transmission car). I didn't know why but I switched to automatic transmission and didn't have trouble after that.
About 4 years later it started affecting my ability to lift my leg over the top bar. I wasn't sure why that was until I searched on Bike Forum and (finally) put 2 and 2 together - THANKS TO BIKE FORUMS. I did the "lay the bike down and step of it" routine of a year or 2.
Last couple of years of riding (with the pain) I was less able to keep my left knee straight over the pedal. By the end I was peddling with my left knee sticking out and that started causing knee pain since i use clips peddles.
Doc said I was a good candidate for hip replacement "whenever you want".
Pain was never so bad I could not manage it with occasional ibuprofen. Really never did that very often but was having more trouble with it by the Fall of 2013.
Got a cortisone shot (one time only) in November 2013 so I could walk around on a trip we were taking.
Unbelievable relief. Scheduled the hip replacement immediately (late Dec 2013 during holidays).
Started riding the trainer as soon as doc said I could (I think it was 4 weeks of maybe 5).

I would't say that I necessarily should have done it sooner but I would say that the cortisone shot was the thing that made me see what I had been missing.