How much time/effort should I put into writing a reference letter for an application for a Master's program? I taught a large introductory undergraduate course
in the most recent Fall semester (Sept to Dec 2017).
An exchange student who took the course
asked me to write a reference letter
for her application to a Master's program.

I spent a bit of time looking through the webpage of the Master's program,
and I found the following information:


  Step 1 - Online Application
  
  The admissions process starts upon submission of the online application along with the relevant documents.
  
  At this stage, the following documents will be requested:
  
  
  (other documents not relevant to me as a reference letter writer)
  Any other documents in English that you think may enrich your application, such as a motivation letter or reference letter
  


Based on the information that I found,
it appears that a reference letter is not required,
and is only marginally helpful.
Because I am busy,
I don't want to put any more time into writing the letter than needed.

Questions:


How much time/effort should I put into writing a reference letter
for this former student's application for a Master's program?
Should I feel bad if I send her a quick and simple letter?
Should I feel bad if I send her a quick and simple letter?


Yes. I cannot stress enough how much this can hurt your student. 
I once had a professor agree to write a letter for me, and I learned afterwards that he wrote "Kerkyra was an intern in my lab for 6 months. She's great, she got along well with everyone and did a good job." (I'm almost not exaggerating.) This basically reads as "This person worked with me for 6 months and all I have to say about her is that she's a nice girl", which is terrible.

Which brings me to your main question:


  How much time/effort should I put into writing a reference letter for this former student's application for a Master's program?


You should spend the time/effort that you feel like spending. 

If you remember the student and have something to say about her that you feel could help her application (Did she stand out in some way during your course? Did she perform particularly well, or ask interesting questions?), it's nice to do it. It does not necessarily take long to write an enthusiastic letter.

If you don't remember her, or if she was an average student, it's perfectly okay to tell her that you don't know her or her work that well, and don't feel that you could write a helpful recommendation under these circumstances. Since her recommendation letter is optional, no letter is way better than a lukewarm or empty one.