As often as you like. The only limit I know of is the one for the first 15 members.
If by boost prestige you're referring to converting inf to prestige.....well I've done 3-4B in a day before and don't know of any limit in times per any time unit. It is slow on updating sometimes.....but I've never had it not show up eventually.
How many times in day/month/week can we boost prestige.