Not all that much. You do need to do a bit of customization for each broad class of institutions to which you’re applying, but you don’t need to heavily customize your application for each individual institution. For details, read on.
UPDATE: See also this follow-up post, which includes poll data both on how applicants customize their faculty job applications, and how search committee members prefer those applications to be customized. In light of those data, I’d now slightly revise the advice below to recommend a bit more customization, especially for applications to places other than big research universities. So long as you don’t sacrifice number of applications; applying to fewer places in order to free up time to more heavily customize your applications is not a good trade-off.
Here’s how much customization I did, back when I was applying for faculty jobs regularly. I’ve been around a while and I’ve sat on search committees and spoken to colleagues, so I’m confident that what I did was, and remains, common in ecology. But please do chime in with your own comments.
I was applying to two types of institutions: research universities (mostly R1s and R2s), and selective liberal arts colleges. I used the same cv for both, but I had slightly different versions of the research and teaching statements and cover letter for each type of institution. I did little customization for individual institutions.
- My teaching statement for liberal arts colleges didn’t talk about my approach to mentoring grad students, obviously. But it described my undergraduate teaching philosophy and experience in the same way. And I had one short paragraph that I customized for each institution, saying that I’d look forward to teaching courses in areas X, Y, and Z. That paragraph was mostly just based on what was in the job ad–if it said they wanted someone who could teach X, and I could teach X, I’d say that. Sometimes, for liberal arts colleges, I’d glance at their course offerings and if there were any obvious gaps I thought I could fill, I’d say so. For instance, if they didn’t have any stats or modeling courses, I’d suggest that I could develop one (carefully phrasing it so I didn’t imply any criticism of their existing course offerings).
- My research statement was the same for both types of institutions, save that for liberal arts colleges I added an extra paragraph about how my main study system (protist microcosms) was well-suited to undergrad research and explaining how I’d run an undergrad-based research program. Sometimes, if I really did foresee collaborating with someone at the hiring institution, I’d add a sentence saying as much, but without any detail. I certainly didn’t spell out customized, institution-specific research plans for each institution to which I applied.
- I applied for a few microbial ecology positions. For those, I customized my research university research statement a bit further, tweaking my future research plans to include some that sounded like actual microbial ecology (honestly, I doubt I was very convincing. I am not a microbial ecologist at heart and I’m sure it showed to search committees.)
- The cover letter for liberal arts colleges had a paragraph highlighting my liberal arts background and how I’d look forward to giving back to liberal arts students as a teacher. That was to convey that I knew what was involved in a liberal arts faculty job, and didn’t just see a liberal arts job as a starter job.
- Had I also been applying for teaching positions with no research expectation, I’d have come up with a second version of my cv with the sections reordered to put teaching-related sections before research-related ones. And I’d have included a paragraph in my cover letter explaining why I wanted a teaching position even though my cv showed I was research-focused. (Of course, had I been truly serious about getting a teaching position, I’d have had a different cv, because I’d have gone out of my way to acquire more pedagogical training and teaching experience, at the expense of my research if necessary.)
- This level of customization did not take very much time, above and beyond the time required to prepare one application packet.