(I wanted to write more than the comments allowed, so I marked this as community wiki so that I don't feel guilty about getting reputation for it).
Here is the advice I give to postdocs when they are trying to figure out what projects to work on and what publication rate is appropriate for their goals.
People tend to judge you based on some combination of your best work and your total paper counts. The better the mathematician, the more they will judge you by your best work as opposed to number of papers. So it is better to write a few strong papers than lots of mediocre papers.
However, it's much better to write mediocre papers than to not write strong papers! In other words, you need to be aware that people are going to judge you based on what you produce during your postdoc. If you are doing a standard 3 year postdoc, then you will be applying for jobs during your third year, so you have exactly two years in which to prove yourself. You need to have things to show for those two years!
The upshot of the above is that you should have a mixture of long-term and short-term projects, but during your postdoc you should concentrate the majority (though not all) your attention on projects that you can realistically complete before you go on the job market. And you should also work on multiple projects at once rather than concentrating all your attention on one goal (which might or might not pan out).
As far as figuring out how productive you need to be, I advise you to do the following. Make a list of people who are in your field (different fields have different rates of publication) and who have gotten tenure-track jobs at places you aspire to be at in the last 5-10 years. Next, go to their webpages and look at their CV's. You want to have a publication record (both in terms of number of papers and in terms of quality of journals) after your postdoc which is similar to them when they were on the tenure-track job market. If you are way off, then you might want to reconsider your career goals.