I think a full-time mother is a mother who has physical responsibility for the child at all times.
I would qualify myself as a full-time mother. If I want some time to myself without my daughter, I have to pre-arrange it with someone else. There is no-one else in her life who regularly takes care of her while I do other things.
Obviously I am not physically with her every minute of the day. But, since she is 2.6, I cannot leave her at home alone and, obviously, outside the home she needs to be watched every second.
Obviously all mothers are 100% in the sense that they are also 100% daughters, sisters, cousins, aunts. But the biological relationship is a separate one to the nurturing relationship (as all adoptive parents will know).
Mothers who work outside the home are 100% biological mothers, but they are not full-time mothers if they have regular nannies, nursery care or any other form of childcare that frees them to go to work. Some SAHMs are not full-time mothers, either, if they have nannies to leave their children with