I spent 50%+ of my time indoors last year and saw great gains/results until my accident. Now I'm back on. Have you considered rollers? With a resistance unit, I can do VO2s and FT workouts on it.
The nice thing about indoor workouts is that you can do targeted workouts--I can be on and off the bike doing 2x20's in less than an hour. A solid set of pacing intervals (Friel's pacing intervals also referred to as STUCI's in the Training Status thread) take me about an hour and 35 minutes. VO2s, less than an hour. When I'm doing 3+ trainer workouts a week, usually it is a combination of pacing intervals and 2x20s. If in race season, I put in VO2s. My longest time on the rollers was 2.5 hours. 1.5 on the trainer (I'd be hard pressed to go longer than that, I think).