I've easily engraved thousands of 316ss/304ss parts each with on average 100 characters. The best tool I've found for this is an Ingersoll "SPOT-IN" 118 degree spot drill. Run at 8000rpm, feedrate anywhere between 10 and 20 IPM. Try to keep the plunge feed less than 10 IPM. Depth somewhere around 0.005" will be good.
If you look at Ingersoll's list of inserts for these spot drills, they sell some triangular inserts which are supposed to be great for engraving. They are, but not for 300 series stainless. Stick with the 118 degree style (square insert) and you will have good luck. The other good thing is that the tool also makes a great spot drill for drilling, and can also be used to chamfer holes and edges. You should have no problem engraving well over 1000, 1/4" tall, characters before needing to index. Just keep the coolant flowing!