Sir Terry Leahy bows out of Tesco on a high. Tidjane Thiam, riding at an all-time low, is digging his heels in at the Prudential. The pugnacious Willie Walsh at British Airways and BP's Tony Hayward – perhaps, like King Lear, more sinned against than sinning – are also under pressure. Just when should a chief executive call it a day?

Failing to deliver on strategic aims, either for expansion or anything else, are ample grounds for immediate marching orders: sorry Mr Thiam. A CEO associated with botched plans can only justify staying on if he (as it usually is) launches a speedy and successful Plan B. Otherwise, time to call in new blood.

CEOs who inherit a basket case or who hit crises on their watch can demand a little more forbearance from shareholders. If Mr Walsh can overcome the unions without breaking BA – admittedly a tall order – he can bow out on a wave of short-term unpopularity but kudos in the history books. His immediate prize: being shoved upstairs to some less taxing post at the combined BA/Iberia airline, where he never has to look another shop steward in the face again.