The long answer:
Overclocking doesn't kill graphics cards - Heat does. Even having artifacts/crashes hasn't been proven to affect longevity.
If you are not hitting the thermal limit on the card, then it should not affect longevity. If the card is constantly thermal throttling, you either need better airflow or need to keep it at a lower clock speed.
Theoretically speaking, overclocking a graphics card or any other component can affect longevity, but it has never really been noticed in practice.
The rule of thumb for overclocking = thermal limit.
= If the card is NOT hitting the thermal limit with no overclock, you are safe to overclock the card.
= If the card IS hitting the thermal limit without overclocking, you shouldn't overclock it. Or, you should look into better airflow/water cooling.
Of course, every card has it's limits. You will not get a 1070 to match a 1080 Ti in performance no matter how well you cool and overclock it. But as long as you keep the card under it's thermal limit, you should be fine.