Hi. I've really been struggling on the fast licks in the solos and I just wanted to know how would you practive difficult kicks and how long does take for you to perfect them?
Metronome, start at a comfortable BPM, then gradually increase the speed, what I usually do is try and play the lick slowly, making sure each note is perfect, then see if you can do this 10 times in a row, if you screw up, start again, until you can, then increase the BPM up more and more each time. Also break it up into sections so you are not having to memorise too much, its the muscle memory you are building up after all.
Slowly t a metronome is the best way to learn in. Play it at a slower tempo till you get it down. then speed it up faster than the song itself, and if you can play it faster than the song, then you can play it at the right speed. If you can't, you know which parts you have to work on. As for perfecting something, it depends on your skill level and how your brain works. Some people naturally learn faster than others, it just depends on how you learn. Depending on how difficult something is, I can usually learn a song's rhythm and the at least general realm of where the solo is being played in about a week or two.
Just take it slow and learn it right. Learning something wrong can make it difficult to correct later on.
What you can also do is learn the solo, figure out what the rhythm is doing during the solo; and experiment with the scale it's using to start improvising, it will help you develop those skills as well.