If it was in the warmer the whole time after it was prepared, yes, 1hour.
If it was (like what my mother in law did) put in a bowl of hot water, then 30mins.
Actually, it also depends on your baby's tummy.
My elder one cannot accept milk that are not 'freshly prepared'.
Tummy pain, diarrhea, vomiting... will start when I gave him 'sit' milk.
but my younger one had no problem at all, even with milk that have 'sat' for 30mins.