There's not a well-defined "schedule", but they've always indicated that the offers would be discontinued after a month. Since they've moved to biweekly updates for PlayStation Plus, they'll probably roll off after either 4 weeks or 6 weeks of availaibility.
For what it's worth,
the PlayStation.Blog PlayStation Store update from 8/3 gave a heads-up warning regarding what's being taken down shortly:
Hopefully, they'll continue to do something like this, so Plus subscribers (as well as people thinking about signing up for Plus) will know when the offers expire.