How long does it take to get your bill if you bought a airplane ticket ?

You pay for the ticket as you buy it, yes, even online. You should have had to enter a credit card number which will be used to pay for the ticket.

You don't get a bill, you pay for the ticket on the spot

Different airlines have different policies. Some charge your credit card right away. Others may not charge your car until a few days before your flight. I have not noticed any specific rule.

Unless you used "bill me later", you pay for the ticket upfront. Some airlines allow you hold a reservation for 24 hours