As an avid bicycle racer and long distance rider for about 25 years, including some indoor training on rollers during the colder months, I can say that how long you work on your exercise bike depends greatly on the bike itself, and how hard you work when you're on it. It's way too detailed a "formula" to come up with for you, so you'll have to simply rely on your own sense of how hard you're exercising. I will recommend that, contrary to what many people do on bicycles, stay in lower gears or tensions and pedal faster. As a rider on the road, I maintained a 90 revolutions-per-minute (RPM) pedaling average, which allowed me the suppleness of muscle to suddenly engage in a massive sprint where my RPM's went up to the low 200's. You'll never be able to attain that RPM level on an "exercise bike", nor is it really necessary, but I tell you that as away to reorient your thinking away from higher tensions or higher gears and thus slower pedaling speeds. As Lance Armstrong and Jan Ullrich repeatedly showed in the world class bicycling race, the Tour de France, the lower gear and faster pedaling was much more beneficial to the racer's outcome, which is why Lance always won, and Jan won only once, and could never win as long as Lance was in the race, and now Jan is not even the captain of the T-Mobile team that he captained for many years. So, you decide how much effort you're going to put out, and keep the gears and tensions low and the pedaling rates (RPMs) high. It helps burn the fat and not build bulk muscle. God Bless you.
