The key words you may want to use to find a useful instrument seem to be "compliance scale," however, the scales I located all seem to be disease-specific. For example, scales have been developed to measure compliance with treatment for hypertension, psychotic disorders, and prescription medication,
The Drug Compliance Scale is the most general instrument I located. It is in the Journal of the Pharmaceutical Society of Japan, 120(2), 224-229 in 2000.