Okay. Going by your estimates.

750 = 6months
800 = 3 months
850 = 1.5months
900 = 3 weeks
1000 = 5 days
1100 = 1.25 day

I've been running mine at 1Ghz+ for 3 months now.
So I guess mine should have died 3 months ago.

It might be true if you run it at that frequency at a constant 100% load. But that's still theory.
Also depends heavily on the voltage and the temperature. Not just frequency.
If you had it like that in blazing hot summer, maybe it'll die way quicker if ran at 100% permanently.

However in real life, it may not even affect the N900's lifetime whatsoever even when overclocked to 1GHz.
In most uses, CPU will be running at 100% for less than couple of minute bursts at a time. (more like seconds)
Maybe in bad cases, up to 15minute at 100% if you run some cpu intensive app/game constantly.
But still, it's not enough to really affect the CPU's lifetime in a noticebaly negative way.

ie. CPU lifetime isn't reduced every time the CPU reaches 600Mhz+. So if the CPU runs at 1Ghz for 1second, it might mean zero negative surplus impact to the cpu lifetime. There is always a threshold it needs to go over before it starts deteriorating exponentially. Also, those TI specs are based on he initial design. Over time, they will get better yields from manufacturing and may actually perform way better than what they initially specced out.