First, a little about trainers. The power curves are very different depending on what type of trainer you have. Mag trainers have a different power curve than fluid trainers. Same is true of wind trainers. Fluid and wind have the most realistic "J" power curves. Mag trainers have a linear curve.
I find my CycleOps Fluid 2 trainer to be a little harder than riding outdoors on a flat smooth surface with very little wind. Last September I did a solo 50 miler with no stops on a slightly windy day with an average speed of 17 MPH when I weighed about 243 lbs. So far this year on the trainer I've only managed 16.7 for a 1 hour Time Trial effort and 17.4 for a 30 min TT effort.
A poster on another thread stated that he can average 20 MPH for 3 1/2 hours in real riding conditions but can only sustain 20 MPH for an hour on his CycleOps Fluid 2. Another guy on this forum, Skydive69, finds his Fluid 2 much harder than real road conditions.
You can probably do a search on this forum and dig up some of those older threads.