The official guidelines tell you to leave it no more than 30 minutes - after that time it goes below 70 degrees, which is the temperature at which any bacteria in the powder should be killed.
The formula manufacturers instructions say to leave it exactly 30 minutes - I suspect because they don't want to be sued if anyone scalds themselves!
According to the official information, if you're going to make up the bottle at any time other than 30 mins after the kettle boiled, it should be earlier. As long as you're careful not to scald yourself, the only other problem to making it up when it's hotter is that the steam makes the powder clog up the scoop.
Some people allow the water to cool to room temperature and then add the powder when they're ready to use it. It's up to you if you want to try that - lots of people do it, although it's not officially recommended.