That goes by race days. From PS3Trophies, other sources welcome:
"Each B-Spec driver reaches the peak of their career at 500 B-Spec days. At this point you will get the trophy and their skills will start to deteriorate a lot faster. To keep an eye on this, you can go to the driver line-up screen (Helmet icon on the GT Life screen). What you want to look at is his career (PAL)/ experience (NTSC) bar. When that reaches 100%, this trophy will unlock. As mentioned, a B-Spec driver reaches his full potential when he has had a 500 day long career. The only way to advance days in B-Spec is to race B-Spec races in career mode."
And if you have more than 2 drivers, you can set one to "share with friends" it stops the career/experience (why 2 names for same thing, stupid devs) line from growing and also freezes their mood. Useful, as I have a lvl 40 whose career line only just reaches the end of Car in the heading... not even 1/4 length.
I retired 5 drivers before reaching lvl 40. I wanted to ensure i didn't do it again.
If you grind a lot with several drivers, they can reach retirement much earlier than 40. My 2 went at lvl 26.
Perhaps, but the drivers of mine that reached their peak tended to have an orange mood/energy arrow almost constantly, so it was a choice of use a driver that has less strength/speed than before and who doesn't really try to win either, or make a new one who will try for at least 8 or so races before going orange (they seem to at least get to lvl 5 before turning orange the 1st time). And if you use max experience giving races, its really easy to get to a high lvl in a short time (even before you get the 2 24 hour endurances), so i didn't miss my retired drivers for long.