2 weeks for nitrite is about right (beginning with new tank etc)
Do I trust Aquarium Test Kits?
It ain’t Rocket Science, those chemistry methodologies are well documented (and standardized etc, etc) and it’s rare to have sufficient “possible interfering” compounds in well maintained aquaria
Most common “error” is user generated rather than chemical limitation of these kits
(I rate expired kits, improperly stored kits as “user error”)
Depends what they are using to do the tests with, as @alto says you have to use the scientific protocol to get a meaningful approach. Nitrite (NO2-) is <"easier to test for then nitrate (NO3-)">, because some nitrite compounds are insoluble (nitrates are always soluble) so you can use <"spectrophotometry"> to test for it. If the LFS doesn't have a spectrophotometer, their test kit isn't really any better than yours.