Hi,
about leaving the medicine longer :
I personnally wait until I feel a decrease of the need to purge (around 10-15 minutes) and the I replace the dots backwards (on the other side) so I can get the most medicine out of the venom. And off course I keep them from drying with spray mist bottle.
After 30 minutes I feel it won't work anymore so I wipe it off and it's dragons blood time.
Maybe I should try to apply it longer ?