when to move on from a song and start practicing anothe

In the early stages, learning is more question of "how" rather than "what," so if you can play it ok as often as not then I'd move on. You can always come back to the bit that caused you problems later.

A :-)

Hey. Im learning how to read music notation from this really easy beginner book. I just wanna know how long i should practice a song before I move on to the next one. it sounds silly i admit. but ive been practiving this one song over and over, now sometimes i get it and sometimes i dont. should i continue to practice until im absolutely perfect? the thing is i muck up on the change from F# down to A cause my hands not strong enough yet. I reckon that can only come with time though. what do you guys think? can i move on to my next song?

thanks
It's been suggested before that if your primary goal is to learn reading, then you should move on once you can play a piece from memory (because then you're not reading any more), and re-visit it a few weeks later.

It helps if you have a large selection of music (at your level) available to practice reading from, so you don't get your entire book memorized, and run into a wall.
hint: check your local public library

If your primary goal is playing (whether reading, or from memory) then stick with one (or 2) pieces until you are almost bored with them, then move along. Don't be afraid to come back to previous songs occasionally.