4oz per tire, generally in a 29x2.25 or 2.4 tire.
I go heavy on the sealant. I've been bitten too many times as "too little, too late" going with less. Sure, it'd be nice to save 1.5-2 oz per wheel in rotating weight by going easy on the sealant, but then' I'd inevitably be tubing up trailside because it dried up or filled cactus holes at a faster rate than expected.
I personally use Serfas TireMedic, it works, it's cheap, always in stock at my favorite LBS and comes in 4oz bottles.