I think around 17 hours? Then it crashed and I reopened it \:D/ Also, since this is not really a "discussion" or anything, this might belong in off-topic? Not sure though~
Its probably just four or five days at max tho, as i log off when a lot of unwanted people keep writing to me. I rather pretend to not be there then just ignore them, to seem less impolite
I tend to not like leaving stuff on for too long because it burns up my laptop, and I usually close things when I'm not using them to stay light on RAM.
My max is probably around 6 hours, maybe up to 8 or something, but it would probably be because I forgot I left osu! open.