I do not know the exact amount of time, but I know it seemed like forever, but using Lion my MacBook was taking a few minutes to boot. It would also run very sluggishly for the a little while after booting.
I got tired of it last night and wiped it and put Snow Leopard back on it, boots a lot faster and runs a lot smoother. I don't care if I lost the $30 for the upgrade last summer, I would rather have a better running Mac.