It depends on the body length of your baby, and they vary so much it would be impossible to say! Just let them out when you feels they are too short. You may find you're on the maximum length as early as 6 months, but don't worry - it's amazing how they seem to stop changing shape so much, and start to slim down, once they start to get active.
Having said that, most nappies are not described as birth to potty, but rather Onesize. Sometimes they just don't quite go through to the potty stage - it just depends on your baby's shape and build.