Hey guys what would you suggest for a pull off exercise? im trying to learn the heartbreaker solo (led zeppelin) and im getting really discouraged because I just cant seem to do trills and pull off fast enough so what do you guys recommend.
and how long did it take you to master a solo or the heartbreaker solo?
First, make sure your technique is right. No tension, consistent sound, consistent rhythm, etc.

Then, just start doing pull-offs in succession at a comfortable pace. However, if I were you I wouldn't just focus on the 1st and 2nd fingers. Look up some pull-off excersises that incorporate all four fingers - not only will you get better at doing trills, but you'll greatly also improve your finger independence!

To answer your second question... How long is a piece of string? There's no fixed answer for those questions. Let alone the fact that we don't know anything about your playing..
the amount of strength/tension needed in executing a hammer-on is greater than the amount needed to hold that note after you've done the hammer-on. same with pull-offs. so try to only exert yourself when actually doing the hammer-on or pull-off, because it takes almost no effort to hold a note down.

keep making sure the trill sounds like a trill though, just because you're thinking so much about relaxing doesn't mean you don't play firmly. recording and listening to yourself helps immensely.
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This is going to sound obvious but is good advice nonetheless: start off slowly with a metronome. Forget about playing along to the song for now, get it nailed at a slow speed until you've nailed it and then gradually increase the speed a little at a time, not increasing the speed until you've nailed the previous one.
One exercise I've done in the past that I thought was effective was something I called "trill combos". Basically, pick a fret, let's say the 7th. You then hammer and pull between 7 and 8 in 16th notes for one bar. Then between 7 and 9, then 7 and 10. Then between 8 and 9, 8 and 10, and finally 9 and 10. Each of these last for one bar. Then without stopping, you go back to the first shape, 7 and 8, and go for 2 beats, then through the cycle with each pattern lasting 2 beats. Repeat. Then switch to each pattern lasting one beat, and repeat the entire cycle 4 times.

This isn't an over night fix (nothing is), but if you give this a little bit of time every day, you'll soon notice the exercise starting to seem easier, and not long after that, you'll start seeing an improvement in your legato across the board.

A few notes about performing the exercise. First of all, if you start feeling excessive tension, stop, relax your hands, and then pick it up again in a moment. Start it slow at first. You don't have to practice it fast to see improvements in your legato technique. And one other thing - watch that you are hearing the patterns starting on the lower note as you repeat them. You'll see what I mean when you try it - there's a tendency, especially when repeating one of the patterns for a full bar, for your ear to start hearing the higher note as the start of the pattern, which will throw your timing. Finally, practice it at different frets, and particularly on different strings (not all in one session - just pick a different fret/string each time you do the exercise)

Anyway, I hope this helps. Good luck!