I was wondering if i can substitute lemon extract for a recipe that calls for lemon juice? or if there is another sub for lemon juice..

I think it depends on what your making. Lemon juice probably isnt as concentrated as extract.

the recipe is for white chocolate cream cheese frosting....i dont remember where i got the recipe though...

I wouldn't measure for measure but I bet you could just dilute it in water with no trouble.

Ive substituted lime juice for it and I believe if you add some water to cream of tartar it would work the same because its an acid. I guess it all depends on what the recipe is for. Ive also left it out before with no harm done on some.

thanks!
weirkd-do you know how much water to cream of tartar

I'd use a touch of lemon essence and a pinch of citric acid in a few tsps of water.

Since you're making a frosting, the lemon juice is almost certainly just for flavor, so you can substitute lemon extract (use quite a bit less... extract is stronger, and maybe add a little extra water or other liquid to make up for the missing juice). Or substitute orange juice, lime juice, or just about anything that would taste good.
If you were making a cake or some kind of baked good, the lemon juice would need to be replaced with another acid (like vinegar, cream of tartar, etc.), because it would probably be helping to activate the leavening.