Im an apprentice and painter.
That depends on a LOT of things, i'll start with the basics.
The price for the work can depend on what is being done, what materials are needed, how long it takes and how much preparations needed - For example, if the walls are in a bad state, they will need filling, and then second filling which can add up to 2 days of extra work to the price.
On top of that, it depends in which way your are being paid - some people will charge you by the day E.G. a fixed price each day until the job is complete, or others will charge a full amount and say in 1 week all the work will be done and you will pay them the amount they quoted you.