I don't set my mash time based on gravity. It's more to do with mash temp and target FG.It seems that when I've seen recipes for higher gravity beers they use a longer mash temp and always equated them. Are the mash temp/target FG factors more common in high gravity beers? I'm always willing to save time.
The more grain there is the more enzymes there are so you shouldn't need to mash longer unless you are using a lot of adjuncts or a very thick or very thin mash.
the enzymatic activity slows at lower temps so if you are mashing at 162 things are going to move along much more quickly than if you are mashing at 148. So if you want a nice dry well attenuated barley wine and you're mashing at 148 then a longer mash time is warrented. In that sense it seems gravity related but if you want a nice dry saison starting at 1.060 and you don't want to use sugar you might also want to mash at 148 for 90-120 minutes. on the other hand, an American Amber where you want a lot of body and richness and mash at 158 you only really need to go for 45-60 minutes.