I used a tablespoon per 5 gallons. My tank was also planted with some basic plants, and this amount of salt didn't seem to have any effect on the plants. With each water change i added another tablespoon for each 5 gallons i replaced (more or less). Make sure you use Aquarium salt, and not just table salt. I believe most table salt has additives in it.
I was adding the salt, along with high temperature and Melafix as an ick treatment. After the ick went away, i just stopped replenishing the salt during water changes, but i read some people try to keep this saltiness level to help prevent sickness.