This time of year I'm often asked about indoor bike workouts on a trainer. I would much rather have the athlete training outdoors, but realize this isn't always possible due to weather and short days. The most frequent question I get has to do with how long these rides should be. The potential benefits of riding indoors are not much different than riding outdoors. However, indoor training is much more mentally difficult than riding outdoors for most athletes. Motivation is likely to suffer. I’d much rather coach someone who is highly motivated but has low fitness than the other way around. So I generally make the indoor rides shorter than what the schedule called for had the ride been outside. And after 2 or 3 back-to-back indoor rides I'd prefer the athlete did something else such as run, XC ski, swim, snowshoe, off-road ride on MTB/CX bike, etc. So with this in mind, a general guideline based on periodization that I use for the longest weekly ride indoors with those I coach is as follows…
Base 1: 1-1.5h
Base 2: 1.5-2.5h
Base 3: 2-3h
Build: full duration as if outdoors
If the above is longer than what the rider would normally be doing at that time of year (sprint-distance triathlete, crit racer, etc) then I may still shorten the ride below what is suggested above. Again, motivation trumps fitness.