There's been other threads about this (search "tipping"), but it really seems to vary a ton based on time zone, time of day, network, etc.
Generally in the early morning (like 6am EST) I can rack up about 1K coins, and then just 2 every hour. Sometimes in the middle of the day I'll be able to get several ads in a row (if I wasn't at work).
So there's no specific timeframe, but in my experience there's about an hour delay before I get the 2 small ads refreshing.