Builders should generally mind their own business. It is not the length of time, within reason, that matters, but if the work will be right.
Most builders usually charge plenty and take a cut on other trades work and materials, plus they do each job in their own timescale to the detriment of other trades, but yet they often object to plumbers charges and hours taken for jobs.
If the plumbers workmanship is good, then he deserves paying. If you went to court and told a judge that the plumber should have done the job in less days, the judge would ask you how you come to that conclusion and what plumbing qualifications you have. Judge would require a plumbing expert to represent you as you would not be deemed capable of knowing how long plumbing should take and also any contract if one existed. You still would highly likely lose.
I often take an embarrassingly long time to do some jobs, but on the jobs I didn't slacken, the hours were obviously needed for my standard of workmanship.
