You can expect around +/- 0.1 to 0.2mm 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.
For additional data and information, please see this article https://sienci.com/2022/09/09/stiffness-and-deflection-testing-on-the-longmill-mk2/