A: Most co-ops will keep your application on file for 6 months, then shred, so you should be sending in a NEW application every 6 months.
Co-ops do not generally have "wait lists" - this would tend to indicate your priority on a first-come, first served basis - instead, when a vacancy comes available (and this does not happen often), all applications from the past 6 months are reviewed. The co-op's membership committee may start with 20 applications, but only 5 may be suitable for the available unit based on family size, composition and finances. Those 5 will be asked to visit the co-op for an in-person interview... or some committees will even ask to visit you in your home.
Please keep in mind that your application is being reviewed by community volunteers - do not knock on doors or harass them by email or phone.
If you've been applying for a long time with no results, you may want to confirm your eligibility for the unit you're apply to and adjust your application accordingly.