They last 5 years. While you are signing up for your replacement, you might want to note what they don't cover.
Eg if you end up in hospital and can't get home on a commercial flight then the air ambulance is either coming out of your travel insurance or your own pocket.
My son needs one for a school trip to France. The online app form asks for his NI number or NHS number. He's 12 so he doesn't have an NI number. Where can I get his NHS number from (assuming he has one) ?