The coils 'last' until you don't like the vape coming from them any more (usually either bad or significantly muted flavour). They don't generally pop and stop working. This means you are in charge of when to change them out, guided by your own tolerance for the vape.
Some folks swap the coil unit every second tank fill-up, others do it weekly as a habit, some others claim they can go for a month before making a change.
Things affecting the useful life of the coil unit include juice thickness (high VG juices tend to gunk up the coils faster), juice sweetness (ditto for sweeter juices), and frequency and duration of vaping sessions. That's a lot of things to consider, I know, so best just to swap out a coil unit when you're unhappy with the vape quality compared to when you first put that unit in.
Happy vaping.