I'd be surprised if a zojirushi didn't "keep warm" at or slightly above 140F/60C.
The rice cooking cycle will kill most/all of the harmful bacteria, predominantly
B. cereus. The rice cooker on "keep warm" will keep the rice above the growth temperature of all the common food spoiling organisms.
Thus theoretically, you should be able to let the rice sit for longer than you'd want to keep it.
So, unless you culture and throw in heat adapted samples of bacteria, it is unlikely for you to get sick from eating rice that's been properly cooked and stored (proper storing being either >140F/60C or <40F/5C)
You might want to freeze rice if storing it for >= 1 week in a refrigerator, since as stated by wikipedia, some strains of B. cereus are psychrotrophic (can grow in cold conditions).
Fairly few microbes can grow when frozen (I don't think any of these are a food safety problem though), though there are a few common ones that can survive (mostly in spore form, usually if the rate of freezing isn't high enough).
As OverVolt states below, in many cases there are noticeable indications of whether or not something will make you sick (spoilage). Though it's not always a good indicator.
Unlikely to be your model, but on page 15 of this pdf Zojirushi explains the keep warm function
http://www.zojirushi.com/servicesupport/manuals/manual_pdf/nphbc.pdf
keep warm is supposedly >140F, extended keep warm is @140F