Tummy time

Hi Mommies, How much do you give your lo each tummy time? And how many times in 1 day? My lo currently 5mo 7days, but still can’t really sits up against the back of the chair and sometimes can’t hold her head up with some support too. When tummy time also not really can hold her head up. Should I worry? The doc said she need more tummy time, and she still not ready yet for solid. My lo had seizures history.