I did my dissertation in 3 weeks, other assignments usually 3-5 days, luckily I managed to consistently get above 70% so like the above comment says everyone is vastly different.
I like writing in general and have done so for years and years so I'm good at generating an essay without much effort. (I can say this one good thing about myself knowing I'm bad at most things)
For some reason, I even never minded proof reading and modifying people's personal statements in exchange for nothing like a little wet dog. Never doing that again o.O
P.S - If you have an essay support tutor or some service at your University, defo use it if you can. Structure is so important in essays, content and technique come second to that. If you have a good structure it really helps.