I use my turbo to get me back to a level of fitness suitable to get out for a reasonable ride. (In my case 20~30 miles.)
I ride with just enough pressure on the pedals to know you are pedalling, like going up a slight incline, & at a normal cadence that I would ride at on the flat.
This should also make you feel like you are breathing a little bit more deeply than normal.
The gearing is that which gives enough feedback to know you are doing some good.
How long to do it, for me, no more than 20 min sessions - but I tend to do one in the morning & another in the afternoon.