when I first started running, I walked a while to get wamred up then ran for about 10 minutes, then walked to cool down. Then kept increasing my minutes.
Why 10 minutes? Because that's how far I could run. So I'd say just run till you can't any more - then walk to cool down

May it be 3 minutes, may it be 10 - you have to start somewhere. And from there - you can move on up!