What is the general consensus on graphics cards life once overclocked. 
has anyone every collected data on the subject or have a overall idea?

Some food for thought. There's two main things that kill hardware... heat, and power. Some GPUs have beefier VRMs, others have significantly upgraded power delivery. Those are better designed for the increased power draw from factory + user overclocks above the stock reference design.
 
Heat is something you have to handle yourself. Thermal cycling (the temperature swing from ice cold to the GPUs max temp) causes wear on the solder joints all over the card including the BGA joints under the core + VRAM chips. Minimizing the amount of cycling helps, but reducing the minimum/maximum temperature range helps the most. Incidentally it's yet another side benefit of watercooling... Watercooling is far from any guarantee, but I'd like to think it helps there. 
 
And Cool GTX is also right, stable, clean power going to & coming from the PSU is important. There's a great many factors that affect hardware endurance.