I can usually knock out back to back 10 gallon batches in 6 hours or so, the time savings for me are as follows (I use a keggle MLT):
While mashing batch 1, grind your grain, measure ingredients for batch 2
As soon as sparge is done and you collect your batch 1 wort and begin your boil, clean out mash tun, refill HLT and MLT with strike H2O and dough-in and mash batch 2
Time it so once your boil is complete and wort is chilled, transfer batch 1 to fermenter and have batch 2 ready to go to the boil kettle
Of course clean things as you get done with them to save time