This is very subjective :-D It depends on your brewing method, how fine you grind, the steep time, etc.
If I am making drip coffee for Parsley Lady I grind a little finer than the "Automaic drip" grind that you would get if you bought pre-ground (shudder) and I use about 40 grams for 28 oz. of water. That yields three typical american coffee mugs with a little to spare.
If I am brewing in my pourover for me I use 18 grams at about the same grind with 12 oz of water to make one large mug. Clearly I like it stronger
When traveling and using my Aeropress I use 14 gr to about 10 oz of water to brew and then top off the cup with hot water (like an Americano, sort of).
For espresso, in my big machine I use a triple basket and about 18 grams of coffee for a 3 oz. triple. In my manual lever machine the basket is smaller (49mm vs 58mm) so I use 12 grams of coffee to get a 1 oz double ristretto.
Confused? Me too :-D
The bags that i use hold approx 12 oz (about 340 grams) for a reference.
Here is a good guide to brewing coffee and some suggestions for the amounts to use with different brewing methods. But, it's really up to individual taste.
http://www.sweetmarias.com/brewinstr/grind.brew.php
