In my experience, its not so much a matter of time as in temperature. Mashing at 155 will definitely leave you with a higher FG then mashing at 150. I recently did a IPA that the first time I mashed at 154. It went from 1.068 to 1.012. The same recipe a couple of months later mashed at 149 came in at OG 1.069 to 1.008. Beersmith can give you a guesstimate, but for me it comes from knowing the yeast strain and knowing your system. Repetition is the name of the game for me. At least that is how I'm still learning. You want mouth feel, go for a higher mash temp (stouts and porters and such). You want dry and higher ABV?....lower mash temp.