There are several types of multi-attraction pass, and each is valid for a different time period:
Build Your Own Pass and Package Deals: 30 days.
All-Inclusive Pass: The number of days you bought the pass for. Go To City Card Offers a variety of durations, from 1 to 7 days in most cities.
Explorer Pass: 30 days.
For all types of pass, visiting your first attraction activates your pass, and you have 1 year from the purchase date to activate your pass.