t'was I, not IO540 who first put up the 50 hour figure.
However, IO540 does a lot of long trips to difficult places, through difficult weather. I fly a lot of types, including a fair amount of test flying. So, when either of us says "good", we mean "good", not "adequate".
I disagree with him about 30 minutes preparation - firstly planning skills are important as well and a relatively inexperienced pilot needs to be practicing those not just plugging numbers into a computer and trusting the output, and also regular personal reviews of airfield procedures, checklists, charts, aircraft characteristics - all this takes time and is important.
If I was only flying 25 hours per year, I'd still be safe to fly a PA28 on a reasonable day from one end of England to the other, and to deal with the odd reasonably straightforward emergency, but I'd not be safe doing for example, the testing of a rebuilt Auster that I'm in the middle of right now.
However, I'll be frank and say that in my opinion the JAR-FCL minimum of 12 hours in the second half of every 2 years, is dangerously minimal and nowhere near enough to keep a typical PPL with only a couple of hundred hours safe. They're kept safe by flying clubs that keep a careful eye on these low-recency pilots - but only just.
Going back to the original question, I'd venture that our new PPL trying to continuously improve their flying should probably look to a minimum of 2-3 hours per month - and make sure that includes some stuff that makes them work: new routes, new airfields, practicing stalls, steep turns and PFLs regularly...
G