How is it customary to pay a video editor - global or per-hour? Hope this is on-topic.

It is often that assesing the amount of work required for editting a video is a very hard task. This makes the payment issue a bit complicated.

I assume there is no one standart for this, but still, is there a recommended, customary payment system that helps making the deal fair for both parties?
My experience is that it really depends on the situation and the editor.  I personally have done both hourly and project rate editing.  Typically I'll only take project rates if it is clear how much time it will take (ie, a simpler project), but it will really depend on your area and what particular editor you are working with.

I'd suggest having estimates of what you are willing to pay for either option and then simply ask them which method they prefer and then supply them with that number.  If you have a preference one way or the other, you might also ask them if they would consider the method you prefer.