Most of the people that I know that record their virtual miles manage about 2 or 3mph more on their trainer than they ever manage in the real world
I would say that,s about right for a flat or mildly a rolling course (winter timings) but I find I am a lot slower on the uphills on the turbo and although faster on the downhills it doesn't make up for time lost on the uphills, the times on the turbo are very close to my summer times, I am comparing GPX rides of real routes done on the turbo not Zwift rides.