how often should you do cardio?
Depends on the guy and it depends on how far you want to take it.
I can't give you an exact answer, because I don't know you, and to give an exact answer I'd need to know what I'm dealing with.
For most though, at least a good place to start would be 5x wk 30-45min per session. Low intensity.
Now this is how I do it, not saying you should do it as well, but it's what works for me. I start off with 6wk 45min a day. After a little while I start adding in more sessions, this occurs once fat loss starts to slow down or halts altogether. So obviously, at some point I'll be doing 2 sessions per day, maybe something like 45min in the morning and 30min after I train. If this is enough great, but if I still need more then I'll up the duration on those sessions. if that's not enough, I'll throw in a mid day session...I've never had to add a 3rd session in there, but if I needed it then that's what I'd do.
For you, the above could easily be way too much, I don't know. A buddy of mine who post here every now and then "bigironmike25" last year, he got ripped to the bone doing only 4 sessions wk at 30-45min...towards the end of his diet he was only doing 3 30min sessions. (Mike if you read this and I'm off a little on those #'s, make the changes, but I think that's pretty close) Unlike Mike, most guys cannot get away with such a little amount of cardio, maybe something like 1 out of 100 could pull this off. In Mike's situation, he took his calorie intake down very low though, lower then I think he should have, but it worked out well for him, so there you go.