I at least started almost all the essays in advance (before being verified) so I had some extra time to kill while writing them. To me, it has been very important to personalize each essay. I think there may have only been one instance when I submitted actually the same essay (applied to 17 schools). Even essay topics that were similar always had slightly different wording. Example: for the challenge essay, some schools wanted to know how it affected my future as a doctor, others wanted to know what lessons I learned, another wanted to know what coping strategies I used and who I turned to for help....there were too many specifics to just write one version. Sure, a lot were similar, but I tried to always make sure that some of my language in each essay was similar to the language used in the prompt to make my "answer" part very clear. I also wrote several "why this school" essays that took at least an hour to research details for.
Anyhow, I have been writing secondaries on and off for about 3 months. I don't do one at a time - I work on many and come back to essays many times. The quickest I wrote an essay is 2 days (one day to write, one day to look back and reedit). Although this is definitely taking longer, I have not had the experience some of the other people on SDN wrote about where they go back and read their essays a month later only to not like what they wrote.
This is my first time applying to medical schools and I wanted to put my absolute best foot forward. I am a decent writer, but it definitely takes me a while (and coming back to an essay a week after I wrote it) to produce the best product.
Out of 17 schools there were probably 4 I didn't get to within 2 weeks of getting the secondary. I prioritized schools that are rolling and schools I have a real shot of getting into (state schools). Although I am probably at least a week later completing than most students who reuse more parts of their essays, at my two recent interviews I was complemented on my essays so I am happy I made this choice. Maybe a stronger/faster writer doesn't need the extra time though..