The invoice can be raised on a bi-monthly basis, by the 25th of every 2nd month when the account is submitted.
However, the time of supply provisions get triggered on receipt of payment in advance, for which, a receipt voucher shall be issued. Tax should be discharged on receipt of advances, although invoice is yet to be raised.
(ICAI FAQ PUBLICATION 06-09-2017 -: Tax Invoice, Credit and Debit Notes : FAQ NO. 6)