A general rule of thumb:
20 hours = first hour of ok game.
200 hours = first hour of good (or at least pretty decent) game.
After that it's about:
1 hour = 1 hour of average gameplay.
4 hours = 1 hour of good gameplay.
Why so much on the first hour? Because it takes very long to setup everything: database, actors, items, monsters, battle system, basic story outline, etc. It's actually like 80% of your game there.
Or you can always do:
10 minutes = 1 hour of people gonna flame you to death for monster spamming/dumb design/totally bad but you don't really care cause it's just crappy gameplay