Within how many days must I issue a GST invoice for services?

For a normal supplier of services, the tax invoice must be issued within 30 days from the date of supply. Banks, insurers, NBFCs and certain other financial institutions get 45 days.

For goods, the invoice is issued at or before removal of the goods (or at or before delivery, where the supply involves movement). It is safer to invoice as soon as the service is rendered, since the time of supply and the tax liability can arise earlier, for example when you receive an advance.