I want to install 4x8 sheets of T&G plywood in my attic for extra storage room, but there are so many different thicknesses. There will be extremely minimal traffic on this floor as I will only be up there once or twice a year and for no more than 30 minutes to an hour at a time.
Always be safe now rather than sorry later. For the small difference is cost, use 3/4 inch sheathing or underlayment. There are several good T&G underlayment brands out there, but T&G may be a little over kill for your application. Simple sheathing, CDX or underlayment should work fine.
It depends on the distance between joists or trusses, and the weight of the boxes/items you intend to store there. I personally would not use less than 1/2", and probably at least 5/8".