It's an interesting question. Certainly my stab at 20% is on the high side.
I just looked at an electronic logbook for my time on 2 specific aircraft (a G-reg Seneca and an N-reg C421). The numbers were 14% of logged time as actual instrument and 2% as simulated instrument. I think 5% is too low in Europe unless you simply avoid flying in IMC and don't stay current in the aircraft.