I moved out of my Ossining townhouse at the beginning of last year but it took me a while to get it listed for sale. In the meantime I didn't file a change of address form with the post office as I was going up every few weeks to check on the place and could pick up my accumulated mail. However I eventually got the place on the market last Fall and currently have a buyer. If nothing goes wrong the closing should be in the next few weeks. So when I was up there yesterday I went down to the Ossining post office and filled out a change of address form. When I handed it in the clerk said it would be two or three weeks before it took effect which is a lot longer than I expected. It doesn't make much difference for me as most of my mail is now going to my new address anyway but might be a bother in other circumstances.
Addendum (5/15): It seems I was likely given bad (or perhaps just highly pessimistic) information from the clerk. When I returned 2 weeks later for the final time I didn't have any additional mail in my box so it appears the change of address took effect almost immediately as I would have expected.