I've found that anything helps, down to 30 minutes. Longer helps more, but hours per week is still what I go by. There's a balance somewhere. 10 X 1 hour is better than 1 X 10 hours for a weekly total. But 5 X 2 hours is probably better than 10 X 1 hour. So ride when you can, that's what's most important. Sometimes I'll get up a little early and just put in 1/2 hour on my rollers. That helps too. I try not to go more than 2 days without riding. Holding it to 1 day off is better even if all I can do is 1/2 hour.