You can expect around +/- 0.1mm for the accuracy of your machine. This may vary depending on your material and your feeds and speeds. For general woodworking, the LongMill is more than accurate enough. To put it into context, 0.1mm is approximately the thickness of a sheet of paper.
For additional data and information, please see this article https://sienci.com/2022/09/09/stiffness-and-deflection-testing-on-the-longmill-mk2/