You are entitled to you FMLA leave for medical care, so if you haven't used it up, you don't need to tell them under the law yet. When you've used up all your FMLA leave and any PTO you also have, that's when you'll need to tell them if you're coming back or not. If they are giving you more than legally mandated leave, however, pursuant to an agreement to let you take time and hold your job, then you would need to tell them when you reasonably know that you can't hold up your end of the agreement and come back.More germane to you, if you are disabled BUT could possibly keep working with some reasonable accomodation (such as assistance technology, a switch to a different position within the company that you are qualified for, etc.), the company may be required to accomodate you; therefore, since discrimination against the disabled is not legal, it may be that you can keep working, if you ask for a reasonable accomodation and a reasonable accomodation (nothing too disruptive or expensive) that would let you work is possible. You should consult with an employment attorney to see what rights you may have in this regard.