Freshly pumped milk can keep under 25c for 4-6 hours. As I mentioned, I used Chu Chu bottle which is one way out only and I think the saliva probably won't go in the bottle and thus I will keep the milk for 4 hours normally under air-con. Since my BB will drink 1-2 oz and sleep again in the morning, I will give him the milk again once he gets up.
I will keep for 4 hours as I use chu chu bottle which could only suck out the milk (i.e. only one way) and propably would not contaminated by the saliva. So far, my BB is fine with this.