When you conduct a trade (Buy or Sell transaction) using funds that you already have within your Anchor account balance, the trade completes instantaneously, and your account balance is updated immediately.
When you conduct a Buy transaction from a linked payment method (bank account), the coins purchased will immediately be presumptively credited to your account. Once the payment completes (How long do bank transfer deposits take?) the transaction is finalized and the coins purchased become available to withdraw.