So, if you don't have funds (dollars) in your account to buy bitcoin, you:
1) Start a transfer from your bank, it takes around 5 seconds to complete.
2) Go to the "BUY" tab, click on USD–>BTC, enter the amount you want to buy and click "buy" (this takes around 10 seconds to do, even counting for double checking amount to make sure it is correct).
3) Done! it shows immediately in your BTC balance upon order fulfilled (usually around 5 seconds average).
So, from beginning to end, it takes around 20 seconds.