I thought the berries were on a one-year cycle, but I could be wrong. I'll start recording how many berries are on the bush before and after shutdown. In my games with fishing, they always seemed to get back to the 98/99 level after an overnight or at-work shutdown, no matter how much the bush built up.

I hadn't noticed them picking down to a level under 98/99. i.e. if the bush is wiped clean, then it'll replenish to 30, 60, 90 without any new picking until it gets to 120.