Maternity Bras....

Ch
So I asked my Mom, but she had been excited about going from an A cup to a C cup and couldn't remember exactly when she had to get new bras.. I have much larger cup size (40I) and I am not very excited about them getting any bigger... It's so hard to find bras that fit to begin with and they're not cheap (think $40-60 each).. My bras are already getting uncomfortable and too small but I'm hoping to wait it out so I don't have to buy a new size every month... Anyone have any idea when is a safe time to assume they've stopped growing or how to guess how much bigger they'll get?