First, you probably wouldn't brew 6.5 gallon batches for 6.5 gallon fermentors. It is common to brew 5 or 5.5 gallon batches for 6.5 gallon fermentors - you need some head space for the kraussen. I can do two 5.5 gallon batches one after the other in about 4.5-5 hours with an all grain setup (15 gallon cooler MLT, 20 gallon brew kettle, 15 gallon HLT, two propane burners). I time it so that the second mash is ready to be drained when the first batch is done boiling, cooled and into a fermentor. I guess that I could probably keep that pace up for 4 total 5.5 gallon batches, which would probably take 10-11 hours. This would certainly require some prep work, i.e. weighing and grinding grain, weighing and portioning hop additions, etc. That sounds like a worthy challenge. Now to get rid of the wife and kid for a day.