For most new projects, where a clear scope of work is defined, I'll provide a fixed quote for the work.
For ad-hoc work, maintenance tasks and where the scope of work can't be clearly defined, I'll provide an estimate and bill on an hourly basis instead, updating you along the way.
Hourly billing is done in 15 minute increments, with a minimum of 1 hour billable time per calendar month.
Whichever system is used, this will be agreed before work commences and specified in our contract.