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Road or turbo it makes no difference it's all about how long you spend at a given intensity turning the pedals.
Personally I wouldn't want to do 2 hour plus sessions on a turbo - but in winter it's probably easier to do an hour on the turbo than in freezing rain in the dark.
The gains may also depend on where you are. If you can come out of the house and hit decent riding roads straight away it is of less benefit than if you live in the middle of London where the sheer number of lights and junctions make constant riding near impossible. Hence some on here hit Richmond Park to get in steady riding.
You can generally go flat out on the turbo safer than you can on the road. I've never had traffic risks and issues in the garage. Allows focus on effort rather than staying safe.
I would say that 1hr is easily worth 1.5 hours on the road but 2 may be stretching it. But could be right for inner city folk.