I'm not sure but it could depend on what project is being built. I'm saying this because typically my villagers seem to donate anywhere from 50-150 a day.
However, I'm currently building a wind turbine and they've donated about 12,000 in 9 days. It's very inconsistent. A couple days it will only be 100 or so a day, then it'll spike anywhere from 1000 to a few thousand one day.
I know the wind turbine is considered a "desirable" PWP, it apparently helps with maintaining perfect town status. So I wonder if that has anything to do with it. I'll also say this is the first time I haven't paid off the entire fund immediately after setting up the PWP, so it could also be proportionate to how much you've donated? Or it could just be completely random. I'll post again after my next PWP and see if the trend continues.