Apply too many schools? Prof tired of sending out recommendation letter? For grad school application (US univerisity), we need recommendation from prof. I guess the process is that system send out recommendation letter request to prof's email.

Do you guys know how long does it take to submit these recomendation letter? Does it need to fill extra form while submitting these letter?  

I ask because I want to know how time consuming it is. Should I limit the # of school I apply because it might be too much work for the prof?
My experience is that once the letter is written, it takes about 5-10 minutes to submit it to each institution.  Usually there is a short form that has to be filled out with basic information (name, contact information, etc) and sometimes also a list of questions asking the professor to rate the candidate in a variety of ways.

So you probably don't want to apply to a huge number of schools (for many reasons besides just that).  It's hard to be precise, but I feel like less than 10 is no problem, and 20 is probably too many.