Woodey
I guess there is only one real honest answer to this question - for me at least - and that is 'AS MUCH AS I CAN AFFORD AND THEN A BIT'. Anyone who spends less than this must be wasting it on some other unworthy activity which really shouldn't be allowed.
Let us assume a minimum of around £1,500 and a maximum which is dependent on inheritance, salary, or ill gotten gains.
With a quarter share of a PFA type aircraft and flying the minimum safe number of hours spread over a year you could get away with about £1,000 although much of this is fixed cost so you could ramp up the hours a bit and not create a much larger hole.