That depends on the promo. In my case after the installation, I paid 1299 (advance) for 10mbps 100GB plan, 1000 for installation, and 300 for processing. I made sure to have the receipt.
After 2-3 weeks from installation, I received my first bill via e-mail. There's a letter on what are expected of the first bill like the full month in advance and proration (partial charge which varies depending on your cut-off date).
The billing tutorial is also helpful: Bill Tutorial | Billing | Postpaid | Help and Support
I had a billing dispute and clarifications so I called 730 1000.