It depends on a few things…
A lot of factors can affect file size, including textures, the scale of your model, complex geometry, and the way your file is set up. In theory, the only limits are memory and performance.
For desktop VR
If you have a tethered, desktop VR setup (Vive, Rift, or Windows MR headsets) we recommend keeping your files around 200mb, on the high end. Sometimes an extremely high poly count (~1.5 - 3 million) can affect performance as well.
For Oculus Quest
Standalone headsets like the Quest don't have the same processing power as their tethered counterparts. At this point in time, we recommend keeping your models small. See our article on preparing your model for the Oculus Quest.
Our team is here to help make sure the asset import process works for you, and we are more than happy to test your models and/or get you on an efficient workflow using the models you create. Just hit us up!