Alan,
I'm glad at your age you still get inspired.
I think this really depends on the climate, terrain, and a list of other things. I hear guys out in Idaho talk about running a lion track that is 7 days old. However, that is in the snow. As you know, snow can lock a scent in where it will remain until the snow melts. That being said, in the dry desert, I don't see the same dog, tracking 7 day tracks period. Way too many variables to put a solid # on what cold, med., or hot tracks are. I am a firm believer that you have to be in that situation to determine it for yourself.
JMO!!!!!!!
Josh