Ray Lampe has cooked with Chris Lilly, so he should know. In his "Dr. BBQ's Big-Time Barbecue Cookbook" he says to inject no more than 12 hrs before smoking, and I've heard the same guideline elsewhere on forums.
I'd suggest sometime trying Chris' peach butt injection that uses the juice from canned peaches. I like it better than the AJ based injection used in his shoulder recipe.
By the way, Chris' book and Ray's first book are by far my two most valued bbq books.