How much effort does it take to modify a reference letter for different applications? I am in my early PhD years and encouraged to apply for scholarships/fellowships that would boost my funding. All of these applications ask for recommendation letters from previous supervisors, and while I had a good relationship with my supervisors, I am no longer doing work for them, so feel guilty asking the same people several times a year to submit letters for me (this is after all of my grad school applications have asked for letters from them). Especially (I presume) when some of these awards may be asking for specific qualities/information about me that might require some updating of both the letter's content and format each time. 

Since I have never written a recommendation letter or seen what requests specifically ask for before, I was wondering:


How much variation is 'typically' encountered between different fellowships/scholarships? Do referees need to make major edits between different applications or is it possible to mostly copy-paste the same letter once you have it written for the same applicant? How long does it take to adapt a pre-written letter for each new application? How much effort is it comparable to? Sending a regular email? Holding an office hour? Grading a problem set? Putting together a final exam? 
Is it reasonable for students you have supervised in the past to keep asking for recommendation letters? Does this become tiresome?
If these people believe you are deserving of an award and have already written you a letter, they certainly won't mind modifying it for this purpose.  If they don't think you're deserving of an award then of course you don't want them to write you a letter for this purpose.

The answer to #1 depends on the letter writer and the specific applications/awards involved.  But it is a much smaller effort than writing the initial letter.

The answer to #2 is that yes, this is reasonable. Remember that your letter writers were once in your shoes, and asked people to write letters for them.  One day you will pay it forward.