Unless you're on water, you probably can't reach the max voltages before you'll be thermally limited, but generally 1.55volts is considered the max, while many others will prefer 1.5volts.
No matter what anyone claims, these are guesses. Unless the engineers who designed the chip are willing to make a claim for us, we're left to anecdotal research.
I point that out because you're asking for 24/7 (365) safety. If you're overclocking, there is no well engineered claim to that other than stock.
Thuban is new, so we have little data to go on, but it IS the same 45nm process, with a few tweak, that we've seen in previous chips from a couple of years ago, and I don't know of any reports of constant 1.5volts that lead to chip failure, which is about as specific as we can get.
Every usage of the CPU changes it a little, and the more heat, voltage and speed is applied, the faster the effect. It will take years to make the effect significant, but there are several reports of CPU's which once reached, say, 3.8 Ghz, which can't manage that top speed after about 2 years...reductions of about 10% have been observed.