Any advice please?

Melissa • 26. I love my husband and so thankful for my friends & family. We're expecting our first little one.
Our little one was born at 38+2 and weighed 2.5kg so she is tiny. She is 12 days old now and we were told to wake her every 3-4 hours to feed her because she is too small to wake herself (I'm breastfeeding and she was a natural vaginal delivery).
The thing I want to know is, when do we stop waking her to feed and allow her to feed on demand? Anyone know or have experience? Sometimes she is dead asleep and we wake her to feed after 3 hours of sleep and it feels weird because she was sleeping peacefully.