Bob skills start to deteriorate after 400 days in game. That is to say, when you make a Bob, his skills start to drop after 400 B-Spec races have occurred. This happens regardless of how many races the particular Bob has taken place in. I know this from direct experience, because my first Bob (T. Sato) who I used solely until his 450th race didn't start deteriorating until his 400th race, but my second Bob (H. Miyazaki) started deteriorating after 120 or so races because I supplanted him with 4 other Bobs after he was created who had run a combined 400 races.
For example, if I make a Bob and enter him in 100 races, and then make another Bob and enter him in 300 races, the specs for the first Bob will start to deteriorate even though he has only entered 100 races because 400 total races have occurred since his creation, and the other Bob will be fine even though he has entered 300 races. The level of the Bob in question is irrelevant, it is (as far as I've been able to tell) unrelated to the Experience bar filling up, and it definitely has nothing to do with how you use him in races.