I don't wait for my villagers to contribute, mainly because their contributions tend to be so insignificant relative to the overall cost.
I also have been led to believe that, if only 1 bell remains to be donated, animal villagers will not do so, and a player character MUST donate the final bell. So, it seems to me that the ability for animals to donate really isn't very helpful.
I love my villagers, but they are broke as a joke. I would rather watch paint dry than wait for them to make any significant contribution to PWP's. Though it does sound like a good town vs town challenge. See whose town can raise the most in a month without the mayors intervention
I waited before for the metal bench since I didn't know if I wanted one next to re-tail yet. I think I waited for two weeks and only about less than 2000 bells was donated by the villagers and then I decided to just go for it and place the bench there. I was way too impatient to wait longer than that to see if they would donate more. I usually pay everything off by that day and see the ceremony the next day, much less annoying and time consuming that way.
Also about what marierock13 said, if you do leave it at 1 bell, you will have to pay that off by yourself or any other human character can. Animal villagers will not contribute that last bell, I waited a week before to see if anyone would donate to the street lamp. None of my cheapskate villagers would donate...
I never wait for villagers to donate for PWPs since it takes a long time and you can't build another PWP while you are waiting. I don't know if you can put up a PWP and then TT a long ways into the future to get the villagers to pay it all off with the exception of that one bell or not. When I put up a PWP, I want instant gratification.