How long did it take Mary Shelley to write Frankenstein?

Hello erikbenson and thank you for your question.

"Mary Shelley , and some other English writers when Byron challenged
the group to write each a ghost story. Shelley was the slowest. It
took her two years to write her story. It was Frankenstein ."
http://switzerland.isyours.com/e/testimonials/screenwriter.html

Whilst itīs true that she started writing the book in 1816 and it was
published in 1818, in reality it took her just less than a year (about
332 days) to actually write the book.

"16-17 Jun MWS begins to write Frankenstein." 1816 
http://www.english.udel.edu/swilson/mws/smchron1.html


"14 May MWS completes Frankenstein"  1817
http://www.english.udel.edu/swilson/mws/smchron2.html

"1818 1 Jan Frankenstein: or, The Modern Prometheus (London:
Lackington, Hughes, Harding, Mavor, and Jones) is published in three
volumes. Both PBS's publisher, Charles Ollier, and Byron's publisher,
John Murray, had declined to publish the novel."
http://www.english.udel.edu/swilson/mws/smchron2.html


"1816
  16-17 Jun Mary begins to write Frankenstein. 
  24 Jul First extant written reference to Frankenstein in Mary's
journal.
  15 Dec News reaches Percy that Harriet Shelley committed suicide.  
  30 Dec Mary and Percy marry. William Godwin reconciles with his
daughter.
1817 27 Mar Percy denied custody of his two children by Harriet. 
  14 May Mary completes Frankenstein 
  2 Sep Mary gives birth to a daughter, Clara Everina. 
  6 Nov History of a Six Weeks' Tour Through a Part of France,
Switzerland, Germany, and Holland: With Letters Descriptive of a Sail
round the Lake of Geneva, and of the Glaciers of Chamouni , written by
Mary and Percy, is published anonymously.
1818 1 Jan Frankenstein: or, The Modern Prometheus is published in
three volumes."
http://elf.chaoscafe.com/shelleym/chronology.html  

Thank you for your question.

Best regards

THX1138

Search strategy included:
"Mary Shelley" "to write Frankenstein"
://www.google.com/search?q=+%22Mary+Shelley%22+%22to+write+Frankenstein+%22&hl=en&lr=&ie=UTF-8&as_qdr=all&start=10&sa=N

Excellent work.  I had thought it was written in much less time
(closer to 2 months), but it looks like this answer has the facts to
back it.