It depends on the school. The usual format is for them to maybe fill out a few quick questions, followed by a PDF upload. The form itself should only take a few minutes. When I was applying, many schools send me an email to notify me that a letter has been completed, and when my profs did 3-4 schools in a row, I would get these notifications about 5 minutes apart.
Of course, it might still take the prof more time if they edit a few sentences of each letter to tailor it to the school. You won't see this in the email timings since they might do all of this editing prior to filling out all the forms at once.
If you're asking about writing the letter itself, I think my profs says that it might take them 1-2 hours to write the letter itself.