Your first premium is due 45 days after you elect COBRA. After that, you are allowed to pay on a monthly basis, though some plans permit other payment terms.
You can expect your first premium to be higher than subsequent premiums. This is because there will likely be a lapse of a month or more between the date on which you first became eligible for COBRA and the time you made your COBRA election decision. For example, if you leave your employer on January 1, and elect COBRA coverage on February 1, you will be charged retroactively for January as well as February.
You may not receive a monthly statement for your COBRA coverage. You are still responsible, however, for making timely payments.