Learning Schedule

Hello everyone! I am working to improve my classical repertoire, and there are many amazing songs that I would love to learn, but I find that too often I will start leaning a longer song and then loose interest because I will just practice whenever I feel like it. 

So, my question to you all, is when learning a new piece of music, how should I best break it up to learn, and how much time should I spend a day on learning it.

Thanks, and God Bless

JJ Tripp