I'm using an ancient Performance trainer with two squirrel cage fans for resistance. I started riding last summer after years of not riding. For me I warm up for five minutes in 39x24, another five in 39x21. Then I'll do 3 min 39x19, another 3 min 37x17. Then work back down to the 39x21. Then work up to a 39x15. Times vary, usually in the 40-60 minute range. My main goal with this is to not loose much fitness. Get hot and sweaty to get those little capillaries opened up. I hated the trainer years ago, but now I enjoy the work out. I would say that is a real light interval workout. If I can only squeeze in 30 minutes or so, I do a more constant spinning at lower effort.
We had some warm weather that took care of most of the snow and ice. I was able to get out on the bike this weekend, much better than the trainer.
Any time on it will be a benefit compared to not using it.