co-op board approval

No set standard. Ask the sell-side broker in your particular deal if they can give you a sense of how long the board in question has taken to come to a decision in the past. Some boards meet once a month and that's it. Some will call a special meeting if there's an application awaiting review. Some do both. Yet some others will do it differently.

that's the answer..

Also, keep in mind that many boards do not meet over the summer.

The decision generally happens shortly after the interview, one week.

The longer interval of time is waiting to get an appointment to get an interview. Bigger buildings generally have subcommittes that handle the interview process and appointments happen quicker.