You can add too much sugar, but it's pretty difficult to actually do, unless done on purpose. If you're unsure, a good rule of thumb on sweets generally tends to be about 1 tablespoon per gallon of water. Feed with it, and if the plant seems to enjoy it, you can try stepping up the dosage a little bit. Just don't go too wild, because although it probably won't devastatingly harmful to the roots, it may attract insects to your grow.