how long do you do cardio for

It depends.

If I'm in a class, like cardio kickboxing, the cardio part usually lasts about 45 minutes.

If I run, it is typically with my dogs and lasts anywhere from 30 minutes to 2 hours, depending on how motivated I am.

If I'm on flat ground, I can run for 8 miles or more (believe it or not). I've conditioned myself with the hills that I normally run, since my neighborhood has some topography to it.

I typically run from 2 to 3 miles with the hills though. And after three solid miles with the dogs, all of us are wiped out when we get home!

I am 54 years old I work out 5-6 days a week, I do cardio every day. However, on days I do weight training which is 4 days a week I only do 30-45 min of cardio. On just cardio days I do 60-85 min of cardio.
I break it down in time blocks like: 20-35 min stair treadmill
15 - 20 elliptical trainer
10-30 stationary racing bike
10-12 min treadmill at 3.8 mph with 9 percent incline
I try to get in at least 1 spinning class a week.

It seems like almost everyone who posted is doing a lot of cardio and that's great.... but....

DON'T anyone get discouraged if you can't do some particular amount. I hate it when they say on the news that doctors are recommending 45 min., 3X week minimum or X amount of water minimum, etc.

I think that only discourages people who are a long way from the minimum so they just give up. The bottom line is this: If you're doing some cardio, you're better off than if you didn't do any. If you can build up slowly, that's awesome. If your body won't let you, so be it. You're still deriving some benefits from the activity no matter what. Walking to the mailbox without getting winded would be a huge step for some people. I'll never run a marathon but that's not important to me. Being able to run 2 mi. in under 17:42 is my personal goal. Figure out what your goal is and take positive steps to reach it!