It depends on the difficulty of the song. Take the beginning of Crazy Train for example. Learned the rythym and didn't need to practice it once, because its that easy, but the lead part takes a bit more time, I think maybe a couple weeks.
lol, i constantly want to improve. After i finish a song, i would experience new ideas and styles to write another one. Do i play my old songs, sometimes but not much, unless its my best. you play as long as you want, as long as you have the fun, the time, and the improvement.
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Put on some heavy strings and Jam the night away.
The KMT
I try and play every song I know at least once in a day but most of the time I end up just playing the few I really enjoy alot more then that. I guess thats why I'm not so good though.