The time required for acceptance testing tasks varies with the story and needs to be accounted for in story estimates.
In our experience, acceptance test tasks generally take anywhere from 20% to 50% of development time, depending on how dynamic the application is, how new the technology is, the team's experience with testing that type of software, and other factors.
Either start with a percentage of the programmers' estimate and work backward to fit in your tasks or start with the high-level acceptance tests and add up the estimates for the related tasks.
Do your estimate in whatever units work best for you, such as ideal hours, then convert to the same units the programmers use.
Add the acceptance test task estimates to the programmers' estimates for the stories.