Between 30 and 90 minutes on my resistance rollers. 30' is only for zone 1 recovery rides, which I do year-round. I can't stay in zone 1 outdoors, so I have to use the rollers. My 60-90 minute rides are usually steady VT1 rides in winter, building base while it's cold and nasty outside. I also do pedaling drills like high cadence and one-legged pedaling. In spring when it's raining during the week I'll also do intervals in my rollers, mostly tempo work. Riding rollers perfectly takes my concentration, plus it hurts to hold a steady pace for a long time with no breaks, so I don't find it boring. Painful, sometimes, boring, not.
My rollers are in my shop which is at ambient outdoors temp unless I heat it a little. So I'm nice and cool in the winter, plus I use a 24" box fan. I like to feel like I'm riding outdoors and slightly underdressed. More watts to the rear wheel and fewer devoted to cooling. No time spent washing/maintaining the bike.
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Results matter