what is a "reasonable" safe speed over the stock 600Mhz for this device? i know that "technically", anything over stock is taking a chance, but is there any speed over stock that could be considered "almost safe", without fear that it's going to die the next day?

I have overclocked this device to 700mhz. It has been 29 days so far with heavy usage the entire day, every day.

Working great and no thermal issues so far. Device is operating in room temp about 27 degrees celsius.

How you overclock?

How you overclock?

From winbox
System -> Routerboard -> Settings -> CPU Frequency

My RB951G-2HnD can be stable at 700MHz for 1 month, and will be kernel failure every few hours at 750MHz.

i have tested with similar results, stable with 700mhz, unstable at 750mhz routeros 6.30.4 firmware 3.24 same result with rb951Ui (same cpu)

crashed after a few minutes at 750mhz with ipsec througput test at 100% cpu