It really depends on what it is. I do a lot of stuff in between and often just small parts at the same place and at a time, but well, those that I did sat down for and did work on for a longer period of time in a row took some hours at least. I can't tell how many exactly.
Well.. The stuff I did recently with pc took in a lot of cases about 5 hours. While doing a hell lot of other stuff including posting here, talking on messengers and whatnot.
So well I'd stay with "some hours" as pretty accurate haha.