How often should a septic tank be pumped out.?

we live in florida in a 4 bedroom mobile home. there are 4 people living in the home. 2 adults and 2 children. we try to be careful about what we put in our septic tank. the bathtub and kitchen sink r clogging up and there is a bad odor that comes from the bathtub. the commode and bathroom sink work just fine. the problem only seems to be with the bathtub and kitchen sink. when u use the kitchen sink the bathtub makes a bubbling sound. and when u take a shower the tub drains very slowly and the kitchen sink makes a bubbly sound. when we pour hot water down the drains it unclogs but only seems to be a temp fix. the problem usually reaccurs after my daughter has finished doing the dinner dishes in the evening. we only have the one bathroom and the tank was last pumped out 4 years ago.