By the numbers, 9 spd cogs and chains are not that much smaller than 7 or 8 spd as far as load capacity is concerned. The differece is probbably in the quality level you chose. Regardless of the ammount of gears I will only use steel cassettes. I have had numerous probs with alloy cogs, a lot of which would be corrected by a spider equipped set, but thats too much dough for me.
If the cogs are made of steel and chrome or nickel plated(as a lot of the lower level ones are and a lot of Sram's are) and the chain is XT(if shimano but not my choice) or Sram PC-68 or better, you should be able to get a full season or more out of it. I get a a full XC season and a winter on my chains and 2 seasons on the cogs before I change out(and that is before they are truly worn).