Play the game, time yourself, see how long it takes you. Do some division to find how many points you earned in a minute, then do some more math to figure out how long it would take to get the score you want at the rate you measured. It'll end up being way over a realistic time because generally points are earned faster in later levels of games, so you'll be plenty safe.
+rep for that. That pretty much sums it up, Nentusia.