As has been said, the time frame varies all over the place. From one day (not likely) to one week, month, months, even year(s). However, I think it's fair to say that if you don't receive a reply to a requested partial within six months it probably won't happen. Be sure to read the agents guidelines and if a time frame is stated I'd suggest adding 4 weeks to same before you nudge. If no time frame is statedI'd wait 8-10 weeks and then gently nudge.