So, let's see... She has a spell that might require even more secrecy than normal and comes complete with various temptations for misuse, she doesn't have the power necessary to cast it without the ambient magic energy, and if she ever wants to awaken, she needs to use it enough while that energy is still available to her.
No potential for problems here!
Because I know someone will ask (I know I would want to), Sarah does use up her own small supply of magic when casting the spell. The ambient energy just makes up the difference. As such, she is working her "magic muscles" when she uses it (assuming she's not 100% using ambient energy, which she likely was the second time she used it earlier).
So, basically, using the spell over and over again in rapid succession wouldn't help much, but a few times a day might (assuming Pandora is to be believed).