There are different schools on how you charge, could be square foot or time to complete, but it really depends on what carpet or surface it is and what process you would use. You would have to be a bit more specific to get a most useful answer.
I find now that I have a feel for how long jobs should take. I look at the hourly rate I want to make to determine the fee I charge,
Sometimes I feel I can charge more per ft. But I am happy making the hourly rate I get.