I'm not 100% sure on this, but I believe it is at anytime. If this law is the same as in other states, then it would be at anytime.
From my understanding you can have rounds in the clip, just not one chambered.
Also, I believe it applies setting the rifle on the tailgate, leaning the rifle against the truck, etc...
It does seem a little hypocritical to allow pistols to be loaded but not long guns. Perhaps it has something to do with range of the weapon?