I write lots of letters. Postdocs often apply to 50-100 places, then you write one letter, using a mail-merge program, and the applicant does everything but the letter.
For prospective grad students, it isn't a big deal these days since most of them are online. I don't even change the school name--but use a form letter (dear committee:)---and sometimes add a note if I know someone at the school. It takes me about 3 minutes per recommendation (once the letter is written), so a few more doesn't really matter.