Difference between “на сколько времени” and “за сколько времени”

Answer

Hello! These expressions don't have exact translation into English, but they are used in different situactions. "За" is normally used with the perfective aspect of the verb and it means "to finish an action within a period of time". For example:

Я закончу работу за два дня. I will finish the work in two days.

Он выпил кофе за минуту. He drank his coffee in a minute.

The preposition "на" is normally used with verbs of motion and basically means "go somewhere and stay there for a while". For example:

Я поеду в Москву на два дня. I will go to Moscow for two days (and stay there for 2 days).
Мы полетели в отпуск на месяц. We went on holidays for a month (and we will be on holidays for a month).