Here's a couple of ideas how long:
0.5% battery loss after >30K miles: https://www.youtube.com/watch?...
93% after 75K miles:
I'd be happy going down to say 50% (assuming 200 mile range). I do around 10,000 miles each year, so doing the math, it'll take me over 500 years before I replace the battery. Okay, the sheer lifetime degrades the battery too, but I doubt it will kill it before say, 50 years.
An MOT is just something in the UK we need to have yearly so that a car is legal to drive on the road.
I hope you will now consider that batteries really have improved greatly, and can only better.