I dont really know if there is a minimum" recommended thickness. It all depends on how much you abuse a tent, and your tolerance for some premature wear and the possible need for replacing a tent.
The Seedhouse 2
Big Agnes deserves a nod for including coating informationthat is indeed helpful information. In this case, even the coating for the Seedhouse 2 ($219) is on the light side. But thats also because its a tent that, while well made and well designed, is competing a little bit on price. The Marmot Swallow 2P, by comparison, has 3,000 mm floor coating. But it also costs $339.
(An aside, what we are talking about here is not the thickness of the coating, its how much pressure from a vertical water column the coating can withstand before allowing leakage. The bigger the number, the taller the column, the more pressure the material can resist.)
Obviously, a floor with a lighter coating can save weightall other things being equal. Marmots Aura 2p ($299) weighs about half of what the Swallow weighs. Its also smaller, has a lighter (hence cooler) mesh fly, and uses various other weight-saving strategies. But it still has a 3,000 mm floor.
Myself, I wouldnt agonize too much over it. What really helps is to get a roll of 4 mm clear plastic at the hardware store. Cut yourself a piece that is slight smaller than the footprint of the tent. Then pitch the tent atop thatthat saves a TON of wear as you grind the tent into the dirt while sleeping. And there is a very small weight penalty. Just make sure none of the ground cover sticks out from under the tent, as otherwise it will catch rainwater and funnel it beneath the tent.