Hello there, I'm interested how much time takes to edit a long video? For example, what if I have over 5-10 hour video, I didn't remember any fails but maybe they've been, should I rewatch this? Any tips for saving time in future?

If you're recording from an emulator, there is re-recording which means you can load a state in case you make a mistake, and continue recording from there. This way you get a decent playthrough without having to edit out stuff.

If it's a modern game, I recommend to play through it several times until you're sure you can minimize the deaths in your recording.

That's obvious but thanks. But what if I want to cut boring parts of the game? I want to speed up this process instead of rewatching the video. How to do that? On World of Longplays channel there a so many videos with duration over 5 hours.

You can use tools such as VirtualDub or Sony Vegas to edit your raw video before you encode it.

Ikr, I mean, for example, when you recording a very long video (~2-3 hours), you're failing sometime, should you keep that part in mind to delete it later? Sometimes I forget to delete parts but I don't want to wasting my time rewatching video. How are you guys deal with that? I know, maybe it's a stupid question.

The longplay guidelines has a rule that says this:
“Dying IS OK. We're all human. Just make sure you look like you know what you're doing, not running around playing the game for the first time. Look for walkthroughs on YouTube or Gamefaqs or IGN.com before actually recording.”

In the case of retro stuff it's pretty easy since you use savestates along with BizHawk's re-recording feature to optimise the video and make it 'deathless'. We're not against death scenes in videos but we require at least experience in the game they're doing a longplay of, to make it more entertaining for the viewers.

Wow, I didn't know that, thank you!