timing 6.5TD ?

[ckm]

I have a 1995 6.5 TD, had to replace injector pump,so had to have it timed by a chevy mechanic. he said stock timing is 4 degrees, what I want to know is how much timing can I run without hurting the motor? need chev mechanic to do this because of the scanner he uses to see how far he moved timing.
thanks
ckm