Much like mlee97, I performed a detailed shrink study. I had the resources to be able to check several sample sets after 1 hour, 2 hours, 1 day, 2 days, etc, until they stopped shrinking. (Polypro, 9 days!). That gave me a detailed "shrink curve". Now I can use that curve for production monitoring, process improvement, tool improvements, all that.
For production monitoring, I have an "egg timer"; we pull the parts from the mold, and then let them sit for 60 minutes. That's about as long as we want to wait for results. At that point in time, the parts have experienced about 30% of the shrink. More importantly, I recognize, from my shrink study, that if the Quality Tech measures at 60 +/-5 minutes from mold time, we will get consistent measurements. Any less than an hour, and the results will be too volatile.
To make a short story short, pick a time frame, and stick with it. Even if you have to put egg timers next to the parts. Works like a charm for us.