I think it really depends on where you are.. The worst condition I've been in required 100% DEET. That was the ONLY thing that worked against those baseball sized mosquitoes in Vietnam back in 1997.

Not thinking of the health concerns, the one thing I found interesting was how "accurate" the effectiveness timeframe was.. I forget whether I applied 6 times every 4 hours or 4 times every 6 hours, but the mosquitoes reminded me like clockwork.

The one thing I HATED were the mosquitoes hanging out under the chairs and tables.. Getting bit in the back of the knee sucks..