I've seen "5% to 20%" for the blower 970's.
The NOISE may become problematic before the overclock fails but that's a personal thing. Of course there's the silicon lottery, and how good your case cooling is.
Make sure to read how to properly overclock as incorrectly adjusting the power target can negatively influence performance. Here's a random result I found from others:
Temp Target 80 Degrees C
GPU clock +200 MHz
Power limiter 122%
Mem clock +500 MHz
Voltage + 87Mv
FAN RPM default