I dont really know if there is a minimum" recommended thickness. It all depends on how much you abuse a tent, and on your tolerance for some premature wear and the possible need for replacing a tent.
Seedhouse SL2 Tent
In this case, even the coating for the Big Agnes Seedhouse 2 ($200) 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 3,000mm 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 weight, all other things being equal. Marmots Aura 2p ($299) weighs about half of what the Swallow weighs. Its also smaller, has a lighter (and cooler) mesh fly, and uses various other weight-saving strategies. But it still has a 3,000mm floor.
Myself, I wouldnt agonize too much over it. What really helps is to get a roll of 4mm clear plastic at the hardware store. Cut yourself a piece that is slight smaller than the footprint of the tent, and then pitch the tent atop that. That saves a ton of wear as you grind the tent into the dirt while sleeping. And there is a very small weigh 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.
The 2009 Winter Outside Buyers Guide is now online. Go ahead, get everything you wish you had gotten over the holidays.