lol, 20 minutes sated you have when game goes bad and you have to cover for your team alot because they suck too much to lane properly. if game goes really good, you are sated by around 14 minutes and regularly you are sated by 18 if everything is even. 20 is like THE LATEST you should ever be sated and if its past 20 and you are still not sated, things went horribly wrong.
taking dragon sure helps but its not your main objective as it takes alot of time to solo it early and you sometimes run the risk of getting forced out, giving a free dragon to the enemy. also, it doesnt give you gold/exp so you couldve just killed 3-4 camps in the same time effectively getting an extra level over someone who tries to cheese dragon.
i dont know what champ you play but some can solo herald at level 8+ which is really good for some stacks.