Leash went with zero warning on a head-high one and I had a long, embarrassing swim in. It came with the board when I bought it second-hand — no idea how old it is. Is there a rule of thumb?
Check the swivels and the cord for nicks every few months, and replace after any proper hold-down. If you don't know its age, that's your answer — they're £25. Your swim was free, but only just.