I find customers that want a larger inground pool are looking for and expecting a little more than usual. When they move them indoors these expectations increase and they are also have the means to pay for them and don't mind doing so. Because of higher potential for odor etc. indoors there are other considerations. Its all dead in the water if the circ system isn't up to par.
As above, I would not even think in terms of 8 hr turnover anymore. First thing I do is divide by 6, then 60 and get the number above at 72gpm. Perfect for a 30" sand filter at about 15gpm./sq.ft. Want awesome? Turns out that 4hr turnover is 108gpm which is a very nice match for a 36" sand filter. 2" plumbing on the 30", 2.5" on the 36". Bill mentions 6.5fps which is a nice velocity for suction lines but we can actually go a bit higher on returns without too much issue. There will be 2 suctions lines also, so very low velocity there. Putting a 2-speed pump in and operating on low speed will be very efficient. If I post much in here you will see me say that a lot 
These upgrades would probably cost no more than an extra 2k on a what, 50k pool? The operational costs will be less and the water should be better and easier to maintain. Any issues that occur can be more readily dealt with because there is the capability for huge turnover. Awk! Thats 3cents worth!
Your original question on how long to run a filter system. You need to run it enough to keep it clean. Clarity is easy, if it is debris. It then needs the filter to run. Chemical imbalance is another issue and when it shows up it is too late, now you play catch up. And if it is Algae, you really might have some work to do. The algae will probably be because the chlorine could not get to the spot where the algae is growing in sufficient strength, in sufficient time.
Second question,
"How does one rate the flow of a filtering system, GPM?" The flow in a circulation system would be in Gallon Per Minute, but a filtration system would be Gallons per minute per Square Foot of Surface Area. That will refer to Sand, Cartridge or DE and they all have different flow rates. These rates also vary between private pools and public pools, and between "governing" bodies such as your local Health Department. Sand for instance is usually residentially rated at 20gpm per sq.ft. A typical 24" diameter Sand filter is about 3 sq.ft. so about 60 gpm water flow through it. Public would be 15gpm/sq.ft. or about 45gpm. Cartidge would be 1gpm res and .375gpm public, D.E. usually 2gpm either way. As I say, those numbers can vary.