How long until birth control lightens my period?

2-3 months or sooner. It generally takes about 2-3 months for your body to get used to a hormonal contraceptive. During this time, it is not uncommon to feel bloated, have irregular bleeding, experience increased acne, etc...But these symptoms subside and are much better within 2-3 months.
A few months. Typically, I would tell a patient that in about 3 months periods should lighten and PMS symptoms should improve. Some patients notice improvements right away, and sometimes it takes longer than 3 months. If you are tolerating the birth control with minimal side effects and periods are predictable, I would encourage to wait it out. Just keep taking your birth control! :).