Security Updates- not too often. Whenever they a hole is found, it is patched within at the most two weeks methinks.
System Updates: These are the 10.3.x ones...those come out every few months, making the computer a little bit more optimized and fixing some bug they found. General work.
Huge Freakin Updates: These are the 10.x. Every year or so, but that will slow after Tiger.
Really crazy huge freakin massive blow-head-off updates: These are the 8, 9, 10, etc, updates. Count on one every 5-9 years. Ish.