If someone suddenly has a large sum of money from a lottery or inheritance or gift, how much time does one have to disperse maaser money from that?
I can't find any indication of how long you have to separate the money (and it seems to me that there is no time limit).
However once you do separate the money, if you didn't make a t'nai (condition) at the time of separation that you retain the right to give it at some point in the future as you please, then you are obligated to give the money to poor people as soon as you come across them. If you delay giving when a poor person is available, you violate a Biblical prohibition of delaying gifts (Ba'al Te'acher). Therefore it is wise to always separate charity money with the condition that you retain the right to hold onto the money until you desire to give it away.
Source: Shulchan Aruch Yore De'ah 257:3
From my notes on a lecture Rabbi Breitowitz gave on tzedaka a few years ago (I don't know if it was recorded):
It's recommended, though not required, to empty one's tzedaka funds on a year-to-year basis, using any calendar (fiscal, Jewish, etc.) that suits your purposes.
UPDATE: Rabbi Hershel Schachter mp3 on yutorah quotes Noda Bihudah as recommending the year-to-year basis start on Tu B'Shvat (presumably as tithes of funds would be modeled on tithes of crops).