Well, there are some nice calculations
The testing is a stress test - so fully moving hard into various directions.
While playing, you don't always move it hard to different directions, an analogue stick is there to do small movements, too, and they are not as stressful as full movements.
Also, slower movements don't stress it as much as fast and hard movements.
I think with normal gaming, you have about 1 move every 10 seconds (depending on the game, compared to full hard movements).
Racing games is even less, since you almost never move hard and in a full direction. Same goes for Quake 3, mouse movements or other games - you rarely do hard movements with analogue controls.
For those kind of games, you usually use a DPad.
So, going back to our maths:
If each 10 seconds would compare to a full, hard movement and we will get 500.000 movements, that's about 5.000.000 seconds of gameplay (or 83333 minutes or 1388 hours).
If you think that's not much: If you would play 4 hours per day ONLY with the nubs, that would be 350 days. However, nobody plays 4 hours per day with only the nubs, I guess hardcore gamer will play about 12 hours per week with the nubs (if he plays a lot), so that would be about 115 weeks... and the average player probably only plays 6 hours per week, so that would be 230 weeks (that would be over four years of playing!)
While those calculations always sound like it's not much, it actually is, because in reality, nobody plays as much as we all think
Think about Ocarina of Time: One playthrough is about 30 hours. So with 1388 hours, you could play more than 45 times through the whole game... enjoy
Another thing not to forget: the 500k we're aiming for is the MINIMUM that it should reach. Some might even reach double the lifetime.
It's very hard to do proper calculations here though: Some gamers stress the nubs more than others do... but I guess this is a good rough estimation. A few years with intensive playing should be in for you.
And if you only use it as mouse, it should last WAY longer.