What a question ... as always, it depends.
I would take for granted the 50h per annum minimum flight time as crucial for ownership. Everything below is usually economically unjustifiable. Total costs will start at around 5.000 Euro a year (not depreciating the purchase price), BUT there is no upper limit.
So, we are stuck in examples from fellow flyers and that has the nickety of very few people really calculate total costs. I had to do, because I am using the plane partly to commute and the tax office wants me to present TCO data.
Running at about 120 hours a year on an old 172 gives about 125 Euros per hour operational cost and 185 Euros per hour total cost (purchase and several avionics updates, such as Mode-S and GNS430 are depreciated over 10 years, calculation according to standard commercial framework). This also demonstrates why 50h/a won't be advisable - at 120h non-operational costs are about 1/3, at 50h it would be already about 55% = more then half of your money goes away for letting the plane stand somewhere.