It depends on a ton of factors.

Is your race intelligent? Dumb giants are always going to learn slower, although I heard they improved the low-end of learning a bit ago. Elves are naturally going to improve much faster.

Do you tank a lot? A lot of people who really want good defenses will solo-rank as long as possible in the lower levels.

What level are you? I believe defenses wont go above 80% before level 15 or something. Also, due to the pace at which they improve, it's very unlikely that a lower-level character will have great defenses, unless you've deliberately spent a lot of time working specifically on training them up (i.e. by tanking multiple mobs at a time while doing low damage).

I usually find the best general policy is to not worry about it. Solo rank some if your race/class is conducive to it, try not to do too much ludicrous-speed power ranking, and try to tank as much as you can when in groups. Other than those loose guidelines, just play.

Aarn