Yeah, if you make a 2.5 liter 1.030-1.040 starter with a smack pack, it should be done fermenting overnight, especially if you are using a stir plate. So letting it go 36-48 hours is plenty of time, just cool it overnight (or during your brew day) and you should be good to go. If you're not using a stir plate it will still go quickly, just give the starter a swirl whenever you walk by.