How long after a tick bite would it take for her to develop a rash?

3-28 days. A tick bite can transmit the organism that causes lyme, which can lead to a characteristic circular rash around the tick bite, beginning a few days to a few weeks after the exposure. Early treatment of lyme disease with antibiotics is important to prevent complications of late lyme disease.
3-30 days. The classic erythema migrans (bullseye) rash associated with lyme appears in an average of 1-2 weeks but the range is 3-30 days. However, the absence of such a rash does not preclude the possibility of lyme- no more than half of those with lyme recall such a rash. While infection risk increases with increased time of attachment, there are proven cases when ticks were attached for 6 hours or less.
7-14 days. If you are concerned about lyme disease from a tick bite, the tick must be a certain type, must be attached for over 48 hrs and must be infected before there is any chance of transmitting disease. In that case, it would be about 10 days before onset of lyme rash. Most ticks transmit no diseases to humans. When a tick is removed, there is often a small red spot or bruise from the bite.