how long can you leave a formular bottle before its no good?

I have been breast feeding but my baby feeds so much and so constant I've started making him a bottle after he wakes for his next feed any time after half 9 as I've been told he should go for longer of a formular, but tonight I made it at quarter to 10 as I thought he was stirring to be fed but as I've came back in he is flat out and not stirring, he hasn't drank from the bottle yet so I'm just wondering as no bacteria has had the chance to get into the bottle how long will it be okay for?