A plumber can charge by the hour or give you an up front price. It can take an hour or it can take a day and a half.
Get different quotes but bear in mind that not every job is cut and dry. There can be unforseen issue with plumbing. Like stripping an allen screw. Be up front about it with the plumber. I may cost more if it comes as a surprise.
Also you can check the national average for plumbers pay rates. But that does not mean the Plumbing company or the boss.