My sister lose a very noticeable weight,with a good diet ,which was some plan without dinner,and she used a indoor spinning bike,so if it same as a bike we use in outdoor,so heres what her doctor tell her,if you keep biking for less than 20 mins or 20 mins then the fat in your body will change to muscles,and they will be hard.but if you bike for more than 20 mins thats the good thing for losing weight.
So you gotta do it for more than 20 mins,and you gotta sweat!
hope it helps!