The loss of any teacher for a long period of time through ill-health can be unsettling for both pupils and staff; especially when the person is the head. There are rules governing how long a teacher can receive sick pay.
For a head, it could be as much as six months on full pay and six month on half pay. It may be that the head wants to return to work, but her doctor is unwilling to let her return until she is fully recovered. This sometimes makes for uncertainty, with the absentee telling you they will be returning and then being unable to do so. If you feel that you do not want to continue as acting head, talk to the chair of governors and the school's link adviser to see whether another head might be seconded to the school for the remainder of your head's absence.