I start advertising at the end of the regular season / beginning of the playoffs. That gives me two full weeks to start recruiting before rollover actually occurs.
Your goal should be to have all your new owners lined up to take over for departing owners when rollover occurs. Not always possible, but that's what you should strive for.
As I stated earlier in this thread, quick rollovers are an attractive feature of a world, and over time will help attract good owners who want to stay.