Bounty runes spawn at the start of the game, and every 5 minutes after that.
Power runes spawn 4 minutes after the game starts, and every 2 minutes after that.
Outposts give XP every 10 minutes as long as your team controls at least one of them.
After he is slain, Roshan respawns at a random time between 8-11 minutes.
Creep camps spawn every minute, starting at minute 1 of the game.
All of this is a lot to remember and it's easy to forget to go for runes during the game.
For fully customizable reminder alerts to help you during the game, get the DotaTimer app available for free on the App Store.