I am currently taking 5HTP to help with slight depression and anxiety but would like to switch to an SSRI. How soon after stopping 5HTP can I go on?

See below. You would need a prescription for SSRI and you should disclose all medications, including nutritional supplements you may be taking, to your doctor. It is unlikely that 5HPT is doing you any good.
Regular exercise is as good as medication for mild depression and has other benefits.
Wish you good health! - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form.
Practice safe sex, if you have sex.