Based on the current rate of distribution of non-backup BGs, which is often concluded after 40 or 50 seconds, you shouldn't wait at all. There's no way to predict what BG number you'll get and there's also no way to predict if your phone is going to encounter a glitch and you'll have to close and restart the app. Sure, you could go to a GET CM to get a different BG number, but I don't know if there're any reports of people who've had success with this. Another thing to consider is that there've been days where the ride was down for significant periods of time and people with later BG numbers either didn't get to ride or rode much later in the day than had been anticipated.
In short: don't mess with the BG process if you want to ride. If you want to ride at a particular time, wait a few months or even a year or two, when FP+ will be available for the ride.
good advice. Thx