Cars that I have had that have done 100k miles two needed a clutch and three did not need a clutch.
That is a high mileage for a UK purchased ND so you are in unknown territory I’m afraid. I would have hoped towards 100,000 miles. A quick look on Miata.net only shows people putting new clutches in ND cars that have been used for racing.
It’s not clear if you’re asking how long an ND clutch will last on average or if you have symptoms that suggest you need to replace one. If you have symptoms how do they present?
This is the strangest question i’ve seen on a forum for a while.
You show no symptoms of a clutch slipping or gears hard to engage but yet you’re asking how long a clutch lasts. This massively depends on the driving you do, where you drive (urban/motorway/rural), how hard you drive, if you track the car at all, launch hard, slip the clutch, hold the car on hills etc etc…
They can last hours if you go drifting and kick the clutch or decades if you just cruise on motorways mainly. I had a Boxster S on 110000 miles on it’s original clutch, a 2001 new beetle on 180000 miles on it’s original clutch and a drag strip prepared aircooled beetle that needed a new one every few months.
LOL, you might also want to read it again. I would very much suggest that that post reads far more that that you are having a clutch issue rather than you being mildly curious as to how long a clutch may or may not last.