You didn’t tell us how long. For this scene, I’d say around 2 mins on a mid range machine?
The lightmaps are in channel 1 and should look like UVs. If they look like rubbish, you can remake them using the static mesh editor.
You often get errors about overlapping lightmaps. The error is related to the lightmap, not the UV. As long as you know the lightmap is ok, it’s probably fine.