Digital BW, The Print

I'm in the process of creating curves in QTR for MIS UT-FSN, and I wait at least an hour for 
the test patterns to dry/settle before performing densitometry on them, but I've noticed 
that a day later, the densities have changed slightly (lighter).

My question, then, is how long should one ideally wait before performing densitometry to 
create a QTR curve?

-Steve