Do people forget songs once they master it and don't practice it for months and years? I am practicing many songs and doing ear training and scales and chords and many things in parallel. Hence have charted out a routine for me. But I want to know how long should I practice the songs, how should I space my repetitions such that I recollect it easily whenever I need it. This is not happening as of now if I don't revisit the songs i had practiced earlier and practice it again. Does this happen to all? What should the routine look like for achieving above purpose?
There is no fixed repetition count. You will definitely know once it is embedded in memory.
The stages to embed a piece in memory are roughly:
Play slowly. Play so slow, that your brain can keep up and you neither make mistakes nor need to slow/stop to prevent mistakes.
Once you feel comfortable, speed it up a little. Again stay in a tempo with which your brain can keep up.
Continue speeding up until you can comfortably play at concert speed or 20% above.
Now you already memorized the piece and your brain only comes in at a few difficult points of the piece. Now isolate and practice these points (starting slow again)! Once you got it, expand the portion of the piece to a few bars before and after and practice again.
Now you should be able to relax during playing and think about expression and feelings.
If you can think about what to cook for dinner or drift off with your thoughts while playing, you know you have the piece really ingrained in your memory :). I sometimes like to do this on purpose and try to talk, think or otherwise distract/interrupt me. If I can still keep playing like that, it is now embedded in my fingers.
Excellent answer from Morgler. Your brain is constantly getting new input from all aspects of your life and some things tend to distract from what is already learned, so don't think something is memorized and have it perfect if you stop practicing it. As practice repetition seems to become dull, introduce more dynamics into it. Vary the speed somewhat as you go, to give it a non-mechanical feel. If you are new to your instrument, frequent short practices with emphasis on getting one thing right is better than trying to learn an entire piece at once. "Frequent short" would be 10 to 20 minutes 3 or 4 times a day, rather than an hour once.