Don't ask, tell.
" I need to take 12 weeks for my health." You don't need to provide other details on a personal health mater, except what is necessary to qualify for FMLA, which your doctor's note should address. Check the FMLA rules. IF you feel you need to give detail, don't. Or at least keep it as vague as possible such as my health is keeping me from XXX and I need time to recover.
Don't give them a range either. Give them the highest number you expect to take off and tell them the start date. If after 6 weeks or whatever you feel ready to come back to work contact them then and arrange to come back early.