Definitely let her sleep!

My girls will sleep from an hour to 3.5 hours. It depends on what we've been doing, growth spurts (especially for DD#2), the weather etc. I'm a big believer in letting them sleep for as long as they can. DD#1 has a pretty good schedule that we tend to stick to (although its changing a bit these days with her I think heading down the road of only one day sleep), but DD#2 I put to bed when she's tired, rather than looking at the clock. They go through so much development in the first 12 months - sometimes she can be ready to go back to bed an hour after she got up in the morning, and sometimes not for 4 hours.