Ya the pain is normal. You are using muscles in your hand that you didnt use much, untill now :)

The more you play the stronger your hand and fingers get and you can play longer and longer without pain.

As for how much to practice something. Grab your guitar the next day, without looking at the sheet or lesson or whatever, and try to play it from memory. If it works, then I would say your ready for the next step :)

Personally, what I do his practice over and over untill I can play the song or lesson without looking at the guitar.

Hope it helps

-Seb