Start 20 minutes and gradually increase to 30 minutes and as you're getting more comfortable can progress to 45 minutes and slowly improve.
I think if you're exercising just to be fit n healthy, 30 minutes should be good enough.
But just don't get addicted to it
My stamina also very jialat one like last time I can only run for 10 minutes and need to walk liao.
But now I can easily run 5Km ot even 10Km without walking but with a slower pace and hopefully I can do so for HM.
I used to admire people who can run continuously 30 minutes on a treadmill last time but ever since I started running this year March, I find it's not really difficult. I can easily run 1 hour on treadmill.
It's about endurance and consistency.