Hi there,
This will depend on the quality of the mop head in the first place, we stock and sell both Excel mop heads from Robert Scott along with SYR mop heads, both are leading brands, and dependant on the type will determine how long they can last.
If it's a traditional PY mop head that is disposal, it will begin to discolour almost immediately after using, however ensuring that it is disinfectanted, and not left in a dirty bucket, I would suggest a week to two, another option is the old fashioned smell test, however I would not recommend this, as by the time you notice, most of your customers will have also.
Why not consider a washable mop head, if you are prepared to launder it, this will cost on average around £2.50 however is expected to launder 10 times, so effectivily 5 nights a week, launder it, you may wish not to take it to extremes, given that mops will discolour due to the nature of the environment and that of the chemical.
Cheers
Darren