For normal processing, it takes 2 business days to get the pre-approval letter via email.
In urgent case, visa is processed for around 4 - 8 business hours depending on the time you apply. You can go this page see more details about time and fee for urgent visa.
Note: All visa cannot be processed in weekend ( Saturday, Sunday and Vietnam Holidays), so please plan your schedule accordingly.
The visa desk at Vietnam airport is open 24/7, so you can enter anytime (if you have the visa approval letter already).