Peck Hanson and Thornburn (page 98) addresses "heavily" overconsolidated clays - or strongly, in their words - as having an OCR of greater than 6. From memory, there is no "distinct" point at which the OCR says "lightly" or "heavily".

Right or wrong I typically think of the difference in terms of critical state soil mechanics where heavily OC soils tend to dilate under drained shear and normally to lightly OC soils tend to contract. Then the OC ratio associated with drained shearing at no change in void ratio would define the condition between lightly and heavily OC'ed. That all sounds good and exact but things are rarely that neat and tidy, but I do use this to think about how pore-water pressures may generate during shear (i.e. if a clay is lightly OC'ed and sheared rapidly I would expect positive pore-water pressures to generate and I would consider an undrained analysis).