We recommend using a Tinseth IBU calculator for the exact dosage. Hops are divided into three categories:
Bittering hops that are added 45-60 minutes into the boil Aromatic hops that are added in the last 10-15 minutes into the boil Noble hops that are dry-hopped add added directly to your fermenter
Most recipes would specify the quantity of hops for a 20 Liter batch and the time (it is the countdown time with Zero being flame off or end of wort making process). Depending on the recipe and style you could add from 0.25gm to 5-10gm of hops per liter of beer. However, the variety of hops and degrees of isomerization (due to boiling duration) has a big impact on bitterness a.k.a IBU.