For tourist visa, normally, the visa approval letter is processed and delivered to your email within one to two (01 to 02) working days (not include Saturday, Sunday and Public Holidays) after you completed the Visa application online.
Business Visa takes long time to process, 2 to 5 working days, depend on what kind of your Visa.
Rush case is available in case you want to get the Visa approval letter quicker. But extra fee will be charged.
You are recommended to apply for the Visa approval letter prior your trip 1 or 2 weeks.
(*)Note: The visa landing counter at Vietnam airport is 24/7 open so if you already have the approval letter, you can still pick up your visa at Vietnam airport on weekend.