Question

So when you get burned your attack is lowered. How much is it lowered by....

Answer 1

When a Pokemon is burned it's attack is cut in half (50%)
You also lose 1/8th of your max health each turn
If you have Guts (ability), then it's attack stays the same

Comments

If you have guts the attack reduction doesn't occur and you gain 50% attack boost

http://bulbapedia.bulbagarden.net/wiki/Guts

Answer 2

The Attack is cut in half (50%).