Yo Matt, your helmet is a multi impact design with 4 layers of protection. If you crack the shell, get a new one. if you do a stress test and push in on the sides and it buckles or pops get a new one. basicly look for stress fractures, white lines in the plastic etc. because it is multi layer though it will be hard to inspect the 2nd layer. if it moves around in there it has come unglued and needs replaced. but should be fine. take the pads out and look to see if the inner shell has come undone from the outter. if not its still solid.
I have seen tons of helmets and designs, yours is the best by far, some tards will argue SR or other companys with a kevlar shell, but it is not true, your helmet has the highest dampening to impact ratio of all the brands avalible.
To awnser your question, it is time to replace a helmet if there are visable cracks, stress cracks, or if it comes apart or separates anywhere.