How Many Liters For Each Plant?

A customer has a question and I hope we can get some opinions on it, thanks.

"I have been fertilizing my plants in a one gallon pot and they are ready to be transplanted to a 15 gallon pot.

So here is my question: if you use the amount it says (and I have the starter set which says it is good for 20 plants) how many liters would I put on each plant?

It seems to me that I will be running out of fertilizer with that small amount. Should I only use one litre per plant per week, which will deplete the amount quickly."