yeah, i dunno... it's a tough one...
too many variables i reckon...
i'll throw out 3 months as a minimum.
and also i kinda think of it like this...
if you've got a tap filling a glass, the glass is gonna get more and more full until it overflows. the point where it gets to overflowing is like when you decide you're in love. it'll take a while, but depending on the speed of the water it'll take more or less time for it to start overflowing.
and once it has started to overlow, that's only the beginning. there's a whole sink underneath it that still needs to get filled - but you really don't start thinking about the sink until after the glass is full...
so the initial "i love you" isn't where things are gonna stay forever... those feelings are gonna get more and more, but you may not even realise that there's that much depth available until after something's overflowed