It definitely depends on the pattern - and it depends on how much consecutive time I work on said pattern.
The smaller ones that I've done - such as R2D2 loves Dalek took me a couple hours in one day to complete - where the larger ones - such as the Baby Peach Birth Record took me about three weeks... maybe a bit (and that also was done sporadically in a span of months, but the actual hours I could not tell you).
I've got some bigger WIPs, such as Game of Mushroom Kingdoms and Epic Pokemon Gen 1, which take up some serious stitch time... but I work sporadically on those too. I'm working more on the Game of Mushroom Kingdoms cross stitch because it's a commission (without a deadline) for my best friend who has stated time and again she has no patience for cross stitching. I love it!
It's fun and super cool! My mom used to cross stitch all the time, and I should have gotten into it way sooner!