As long as the teeth don't appear to be growing in crooked or uneven, they are most likely the right length. Ratties don't really need to chew wood blocks (though some like to), as regular eating and bruxing (that wonderful grinding sound they make when they are pleased) should keep them to the proper length. If you'd like to give your rat something fun to chew, try Dentabones:
<http://www.rmca.org/Articles/dentabone.htm>.

Here's an article on rattie teeth if you'd like to check it out:
<http://www.rmca.org/Articles/gnawing.htm>.