I did the Pretzel last week, and I think that's about as much as I ever want to do on the trainer- just under 2 hours and 20 minutes. The climbs were nice in that they had new scenery to keep me engaged, but there's only so much I want to dedicate to sitting in one place and grinding.
Spending the week down in San Antonio riding on actual pavement has left me less enthused for the trainer. 2-4 hours on the bike and then all the wrong foods and lots of beer to offset the gains. I'm going to roll into this season with quite a roll.