Nobody really seems to know, do they? I thought I'd put my language skills at your disposal, so I've been trawling German and Scandinavian websites- suggestions are made of anything between 15 cms and 30cms+, but it's all very vague and most websites don't seem to want to be drawn on this one. I get the impression that noone has kept one in captivity for long enough to actually find out. It being a hypostomus I would be a bit wary of the lowest estimations.