60+ or thereabouts. Add blue, yellow and black and you're between 50 and 60, add green and you're between 40 and 50. After that it gets harder :)

I would have thought the introduction of themed sets would narrow things down again.

Most of my lego is blue, black, or transparent red. (space police 1) Some is black and trans-yellow (blacktron)

The transparent red could be really old stuff, pre-ABS Lego, made from Cellulose-Acetate.

The blacktron sets are from the late 1980's.