yes sngle entry means you have to use your visa in a specified duration/time.i got my us visa around oct 2006 and the expiration of visa is jan 2007.
if you're given a sngle entry visa for july 07 and you decided to postpone your exam to dec 07, you have to reapply again for visa.that's how it goes.
for my single entry visa, i was given 3 months. the duration of stay depends on the US immigration when you're already in the US..some were gven 6 months.that i think is the max for sngle entries (not sure) but since my purpose is to take the exam , i was only gven 3 months.