I replace my chainring teeth when they get very sharp & pointy (except the ones that are designed with a lower profile to begin with). If you look at a new chainring you will notice that all the teeth will have at least a small flat top on them. This flat top will totally disappear. Also the U shaped bottom part of the teeth will have a flat spot. Making the U look wider than it was when it was new.

As mentioned earlier -- shark tooth shaped chainring teeth is another way of telling if it is time to change the chainring. Shark tooth shaped chainring is bad and new ring needs to be installed.