10 mins is not long enough to tell if the voltage is stable or not.
I use a combination of Intel Burn test and Prime to tell weather it is stable or not. No i do not run them at the same time.
"presumbly if i get a system crash i just have to increase the voltage back up until i get a stable minumum?"
yes if it crashes because of voltage. The issue with under volting is that the CPU can return a bad value during its work cycle. basicly it will add 2+2 = 5.
having the necessary voltage is critical.
prime and Burn test both will stop if the value is comes back wrong. IF the voltage is too far off the system will crash.
" how long is good to run prime for?"
I run it for at least 11 hours. 24 is said to be best but there is a lot of debate on that issue. Burn test i run for 40 runs on maximum but make sure you keep an eye on the temp of the CPU when running both.
When overclocking and/or undervolting I generally run something that is CPU intensive. That generally involves encoding some video files to MKV format using the x.264H codec. That generally takes around 3.5 hours to encode a single 2 hour video. If the PC does not crash while encoding 2 or 3 movies, then I consider the overclock and/or undervolting to be successful.