At a minimum level, riding a "hard" hour three or four days a week. That "hour" hour needs to be buffered by an easy thirty minute warm-up ride, and then a fifteen minute or so "easy" cool-down period. So, a total of about eight hours a week.
That mimimal program of eight hours a week will deliver noticeable results for most riders in just a month or two. And, in six months, you can see dramatic improvements in your overall physical condition.
I have an age fifty-something friend who rides every day of the year for about an hour or so. Probably a total of around 10 hours a week. He never rides fast or pushes himself enough to be breathing hard (his ashma would "kick in"). But, after several years of doing this, he is in much better shape than most guys his age.