They can take their time at first, and then some will lay like crazy for a couple of weeks, an egg every day, and take a week off. Make sure they have some extra oyster shell available, plenty of water, never let food or water run out -- that can stop egg laying cold and even send older birds...
ISA Brown is a very good layer, and will probably out-lay all others. Your sex-link is just starting, that is a pretty good size for a first egg. She might take a while to get regular, and only lay 4 eggs a week even when fully adult. Make sure that there are no "secret" places she could be...