At many times the biggest issue with business users is their time availability. But, if you have a good Project Manager and Leadership, there should be collaboration between them and your business users and their management to ensure the time is available, this should be done well before the testing period begins. The Project Manager should also be able to define the length of time they would be required to spend testing. As a Business Analyst, I realize the business user's time is very limited. Throughout the requirements lifecycle I try to provide mockups and/or prototypes of everything, get feedback, and make changes where necessary so that when the business users begin testing it isn't so time consuming or the first time they are seeing the changes. Developing a Test Plan is required to set expectations at the time development begins.