The plugin sends the email instantly but there are a few dependencies though
1) PayPal – the plugin will never send the email with the product downloads untill it has received the payment notification (IPN) from PayPal. PayPal does send the IPN instantly after a payment but sometimes it can take up to an hour (this happens rarely).
2) Your Server – the plugin tells your server to send the email but that doesn’t mean the email is sent right that second. Your server can queue it up if it’s under heavy load and send it later.
3) Mail client – The mail client has to query for new incoming mails. This query time interval can also vary.
It’s not hard to find out what is causing the bottleneck though. You can turn on debug (check the “Enable Debug” checkbox in the settings menu) in the WP eStore plugin then check the “ipn_handle_debug.log” file to see when the IPN was received from PayPal and how long it took for the mail to arrive in your inbox.