If you want to buy your plan before you travel, make the purchase no earlier than 30 days before you plan to travel. The expiration timer of your data plan will start when you first connect to a network in a country covered by the data plan, or 30 days after the plan is purchased, whichever is the sooner.
Alternatively, you can buy your data plan when you arrive in your destination. The plan expiration timer will start when you first connect to a network.
Note that data plans will expire before the expiration date if the plan's data balance drops to zero. If this happens, you will need to top-up your plan to continue using the service.