In German, there is a very idiomatic sentence
Das geht schnell.
with the meaning
It takes only a short while.
I've got some good answers relating the scenario described at the end of my question that was originally the only one scenario described by me. But unfortunately the answers are not (completely) applicable for the following scenario as this scenario does not involve any kind of absence:
Say, you are looking for a small girl with long hair. After a wild play the hair of the girl is disheveled and you want to fix it. The little girl is fighting against, so you need to explain to her that fixing her hair (referred to as 'it') will take only a short while. What do you say to the girl?
--- the originally described scenario ---
For example, if you are on the way in a company and want the others to wait for you for something you need to do on your own (say, you have forgotten something and have to return home and to get it), you usually would say:
Hey guys, I've forgotten to take the book with me. Please wait, I'll get it. Das geht schnell.
What do you say in this case in English instead of 'Das geht schnell'?