At Uni, I would go 3 times a week, just over an hour for each session.
Now that I have work, I would aim for 2 times a week and the same duration.
Its hard to say, just go and see how long you like to spend in there because some people love taking it easy and chilling and end up staying for much longer whereas some like to get it over and done it asap.
It really depends on how active of a life-style you live otherwise.
If you're somewhat sedentary, 40 mins seems pretty good actually.
Do what makes you feel fitter.
I try to do around 25 mins per day, usually 4-5 days per week.
Yep I,m very sedentary, thisvis the first time in my life I have started working out properly on a regular basis, been doing 40 minutes of cardio everyday for about 2 months now. Not missed a day. I love going the gym that's why. I hate wasting time aitting at home. Resting days between weightlifting sessions are annoying because it's I wouldn't say addictive but I can't think of another word right now.