the general rule is 30% or more, sometimes a lot more. sativas tend to stretch more than indicas.
it also makes a good bit of difference what light you vegged under and what light you're flowering under. the bigger the difference in K value, the more stretch it will cause.
as far as yield, it's very hard to predict, there's so many things we don't know, just remember, you get better each time...
or you quit and buy weed