It depends on how "hard" your water is. The harder the water going in to the heater, the more conductive the water and therefore the faster the anode will disappear.
And if you are unfortunate enough to have a water "softener" upstream of the tank (i.e. leading into it) then it'll really burn through your anodes in a hurry (6 months) because of the conductive salts that it adds to the water.
Typically, you'd take a peek after 2 years to get a feel for how fast the anode rod is being consumed. Here, our anodes last about 10 years, but we have the (naturally) softest water in North America (less than 10 grains).
A more typical lifetime is 5-6 years.
New anodes are only 20 bucks, so a very worthwhile investment. Much better than going through all the hassle of replacing a leaky tank (which, of course, always happens on weekends)
Most hot water tank warranties read: "only warranteed if the tank contains a *functioning* anode". i.e. they know it will only rust out if there is no anode left.
Hope this helps,
.../j