check the max string weight that the pipe can withstand, and try not to exceed it.
We had to shear a tubing anchor once and had to pull over the string weight, but that should be your guide as to how much stress the threads can handle.
then to answer your jar question, you need to jar your pipe with more weight than your string weight. Like 30 or 40K over string weight (I've never calculated what you need to jar, and I don't know if there is a way to calculate how much you should jar). But, back to my first response, the string weight factors into how much tension your threads can handle.
I've got a cheap & cheerful excel sheet that works out how much to slack of to cock your jars (many people never actually fire their jars at all when they're 'jarring'!), as well has giving you warnings if you're going over the pipe or connection yield points. My next hitch starts in a couple of days so I could send it to you them.
Also, Fearnley Proctor in Aberdeen run an excellent course all about stuck pipe for both the office and the rig site team- different ways of getting stuck, how to avoid them, what to do about it (ie slump & torque is good for differential sticking but not so good for a pack off!)
Is it a directional well ? Optimum strenght to activate jar I worked with was 40-45t (for the first, 'mechanical') and 60-80t for the second, hydraulic stroke. If jarring gives no effect, and your mud is not oil-based, try the oil bath. Pump it in the interval of annulus and leave the drill pipes under its own weight for some 12 hours. Beware possible income if you passed through any gas horizons at 300-600m.
Sometimes an intensive, pulsating flushing helps. One of the ways is to deflate nitrogen (from pressure compensators).
