You don't have to write back. However, if you are expecting to return whenever you feel like it, you may want to email back. She might fill your spot if you don't even indicate that you may want to return. Not to be malicious, but just because they may have a waitlist or whatever. I personally think it's really rude not to reply, but no, you don't owe her anything. I just don't like being "ignored", but that's my issue.