We usually recommend fertilizing your baskets & containers at least once a week, however each fertilizer is different. Check the instructions on the package your fertilizer came in for the amount to put into your container or basket and how often.
If you have a water soluble fertilizer you can fertilize every time you water, this is a fast acting fertilizer so make sure you do not add to much of the fertilizer powder into your watering can as you don't want to make it to powerful for you plant. After 3 - 4 waterings water once without any fertilizer to give the plant a cleanse.
If you have a slow release fertilizer put 1 tbsp. of fertilizer in your container or basket every two weeks. If you notice that your plants leaves are yellowing or lack flowers you can give the plant a fast boost with a one time shoot of water soluble fertilizer.
You can fertilizer your baskets & containers all season long, from May - September.