If you find that you do have this feature, you will find it in your Settings tab, in your Testing Profiles. See below.
You can set the maximum days to be any number for each individual test. Keep in mind that this is literally the maximum days between tests. If you choose maximum days between tests to be 20, that means that if a client tests on the 1st, then if they haven't been required to test by the 22nd, then there will have been 20 days between tests.
Please keep in mind our guidance for how to properly set maximum days between tests to achieve proper randomization.