1. The eVar will expire only if the purchase event occurs. So, if the purchase event didn't occur in a visit, it will be persisting to the next visit OR to all the visits until the purchase event occurs.
2. Expiration here is a Server Side Calculation. From the moment you change the expiration, the calculation changes at the Server Side, and thus it is updated expiration for both new & old eVar value. However, I never validated it.