It is route & frequency dependent. After all, it only occurs when either there is an overbooking resulting in a real risk of oversale in a cabin or an oversale created by a defective seat or the like. That is close to impossible to predict. Then factor into the equation where you stand in the pecking order should a decision be taken to upgrade someone. BA's preference is, of course, to accommodate passengers in their ticketed cabin and to send the F (CW & WTP as well) seats out empty if not purchased.