My greetings! 
When do I know it’s the right time to stop practicing a riff I’m learning? If I’ve been playing it for a while, sometimes I make mistakes during practice and other times I play it perfectly. How can I tell that I’ve really learned it and should set it aside? Should I play it correctly 3–4 times in a row, or is there another way to confirm it?
Thank you!
Angelos,
Being able to play correctly several times in succession is a useful yardstick. Also consider how frequently you make a mistake. Try and understand the root cause of the mistake eg is it trying to go to fast, is it not fully internalised, do you lose concentration? Then maybe look to address something that may have longer term general benefit.
That said, also keep your practice fun and I try not to get too obsessed about perfection. If the mistakes are the exception then may be time to cycle it out of the practice routine, particularly if you want to focus on a new thing you have been learning.
Then spaced repetition is a useful approach to consider. For example, for a little while skip a day or two and then play it again, then perhaps once a week for a month, then once a month. Structured people write these schedules down and track them. Not necessary but if that suits your ways then it is an option.
Thank you very much David. I needed this. Your help is really helpful
My pleasure, Angelos, try to be helpful and encouraging and always glad to hear when it is as intended.
@angelakis I agree with most of what @DavidP wrote.
If you want a slightly different way of thinking about it, I’d say think about the reason you are playing it.
Is it to get better at playing guitar? I’d say move on. You’ll learn more and can always come back to it.
Is it because you want to record it? Just record it. You don’t have to (and probably won’t) get it right 1st time. But you will get a good version of it.
Is it because you are going to play it live. I’d probably spend as much time as you need to, to be confident you’ll get it right. It doesn’t mean you will get it right, but that’s OK, it’s live music and stuff happens. If you don’t get to the stage where you are confident, just fret chords and pick individual notes. It will sound good enough for most audiences.
From those 3 you can probably get a good idea of when to move on.
Finally: don’t practice it so much in one session that you give yourself an injury.
Thank you very much Simon! 
You ll help to figure out what to do in many situations with the guitar.
Basically i am not so good at playing a gig yet. I am in the middle of the 3rd grade. I am a beginner. I mostly want to learn complicated riffs and to be confident when i am playing them. The thing is that i am playing them sometimes good, sometimes not very good, and sometimes like “my brain is out of order” 
So i think the best is to leave them for now and keep on learning other stuff.
Thanks a lot!
The book “learn faster, perform better” had a guideline that I found useful.
If you can’t do this, slow the tempo until you can. Then keep practising this way, slowly raising the tempo until you get to performance tempo.
This is very interesting! I need this book ASAP 
Thanks a lot! I will try this method definitely! 
As a beginner you should be learning songs not riffs in my opinion. I see no value in learning a riff if you don’t know the song that goes around it. There’s no role in a band for a guitarist who only plays riffs!
Hey that’s very good advice and so easy to forget! Learning riffs is fun but without a song they’re rather useless. I’ve been stuck in the riffs rut. Time to find the song methinks. Thank you.
+1 what @simon_plays_bass said.
Some riffs I work on enter the backburner after a while then come out every now and again, even with having not played them for a while, your general playing ability improving will improve those riffs too.
Plenty of electric guitar songs are based around riffs. Maybe even most rock/metal songs. The song is a collection of riffs for each of the parts.