Typically they are given their mail on a daily basis, provided they have time in their training. During boot camp your boyfriend will have 'personal time' which is sometimes as short as half an hour if they have been behind with training.
Personal time is for showering, laundry, cleaning their wall lockers, and personal grooming as well as writing letters so as you can guess time is short.
Take joy in any letters you do get, but know that he will be happy when he gets mail from you, even if he doesn't have time to write back.
I got tons of letters from friends and family but only had time to send out two or three.