I just tossed out 30 because it's a nice number and seems reasonable. You are right however that 45 minutes is the standard. I think you'd actually see benefits from doing high intensity cardio for 30 minutes however. At the very least it's so much better than 7.
At the end of the day, if 7 minutes of cardio is all you can stand to do, you're screwed.
At the end of the day, if 7 minutes of cardio is all you can stand to do, you're screwed.
The patronising attitude isn't going to help anyone who has gotten that far out of shape.
Even if someone is sufficiently unfit that they can't do more than 7 minutes of cardio work in a session, they still have to start somewhere and build up. The alternative in most cases will be their fitness decaying even further.
Keep in mind that there are a lot of reasons people might be that unfit. Some will just be lazy. Others might be recovering from a serious injury or illness, and just being able to walk for five minutes to get to the shops unassisted is a big achievement.
At the end of the day, if 7 minutes of cardio is all you can stand to do, you're screwed.