How long does it take for the lymph to make a complete trip through
the lymphatic system in humans?

The lymphatic system is not a loop in the way the blood system is.
Lymph enters the lymph vessels along much of their length and
eventually empty into the blood system. So, the time anything spends
in a lymph vessel depends on where it enters. Overall though movement
of lymph is MUCH slower than blood, it might take hours for a sample
entering near the foot to eventually empty into the blood supply.