I'd say whatever suits you. I have a pretty full on training schedule for example next week I have cheerleading on Monday and Wednesday nights (1.5-2hrs each), poledancing classes tuesday and thursday (1hr each), parkour on saturday (2hrs) and a strength/conditioning session on Sunday (2 hrs). I fit in gym in the mornings and try and do about 30 mins each day of cardio with an extra 15-20 mins for strength twice a week. I also stretch for 20 minutes every night.
I found that the more exercise I did the more I wanted to do so if you build up gradually eventually you can find yourself doing a lot
It's also a lot easier if you do it with other people i.e. have a buddy or join a group as it keeps you honest and on track. In terms of machines I use a mix of cardio equipment (though I do favor the cross trainer) since they all work you in slightly different ways and for the wheights machines it really depends on what you want to accomplish - if you want to gain muscle quickly in order to speed up your metabolism then leg machines would be best (not sure what its called but the machine where you sit on a seat and have your legs on a flat surface in front of you so that they're bent at 90 degrees and you push them straight.... yeah its awesome and builds muscle really quickly). If you want to gain strength for something more specific then look for a machine that says it works those msucles (there's usually a sticker or something that tells you what it works. To use myself as an example again, I have to build muscle in my arms that allows me to hold people above my head so I went for a machine where your sitting down with the machine's handles at shoulder height and then you push up so they end up above and slight over your head with arms straight.