The first few weeks, at the end of a 10 minute sprint, my heart would be racing and I would be out of breath for 5 minutes after arrival.
I was amazed after 2 months of doing the same thing I wasn't out of breath, it took maybe 1 minute for my heart rate to slow back down.
EDIT: Now that I've seen the replies, I realize that this is from a very low baseline. I'm personally going to the gym four to five days a week already and my own cardiovascular fitness has not seen much of an improvement in many months.
The typical human body is very responsive when pushed.
If you've never lifted before and you hit the gym consistently and have the proper diet, you will get some serious "newb gains." The first year can be amazing.
But if you've lifted seriously for a decade, it's totally different. Muscle mass is very hard to add now.
Cardio gains from a coach potato introducing a serious routine will be incredible. I went from not being able to run more than 30 seconds to running a 5k in 4-5 months, and that's without pushing myself extremely hard.
The point is about how the body adapts.
My VO2 max as reported by my Apple Watch has been making steady gains, though I'm still far below average for my age.
I didn't train hard, just consistently.
The human body can make incredible cardiovascular leaps when starting from a relatively low point
9 weeks. I have basic fitness but have never been close to completing a 5km run (I think I finally realised I have breathing issues - seeing an ENT soon).