I thought you were suppose to use this everyday to keep it in your system. I thought I read this before in a comments
It is like creatine in the sense that it's stored in muscle tissue.
I'm sure everyday doesn't hurt, and might even yield slightly faster results, but it definitely isn't necessary to see results.
I take it on training days because muscle contractions (along with insulin) help uptake into the muscle. Not using insulin at the moment, so I just shoot it preworkout after my pretraining meal.