you should view the paypal documentation for ipn.
There are also many other threads on IPN in this forum if you use the search.
There is no telling when the ipn post will take place. when the customer is transferred to the checkout success page, the transaction is placed in a queue to be processed with the creadit card company. It can take 2 seconds, or 2 days for the transaction to be processed.
When it id finally processed, the results are posted to the IPN page.
Express checkout can still take the account optional setting.
However, settings for paypal standard like return URL make the account optional setting not function correctly in express checkout.