I think the cleaning procedure is also on mixer instructions manual, but as you asking some thing very clear so:
The mixer should never stops with low athesive level more than essays for hot system(old generation of athesive) and not more than 7 days for cold system.
The mixer head shoul be always closed.
For a good cleaning, first it should be jog out all the athesive outside Better to original tank, flush enaugh solvent in order to clean the remain athesive between mixer tank and head, mean clean the gearbox and pipe from athesive, till get clear solvent coming out from the mixer head.
When this operation finish, it should flush oil from thank to the head, when oil will get out from head, mean replace the solvent, so we can said the mixer is safe to be used an other time without problem.
Customer can clean only the NCO tank, as the OH part never give problems.