Your data expires from the date you purchased your eSIM (check your plan details—typically 1, 6, or 12 months)—you can always log in to your account and check the billing details.
For example: If you purchased on March 1st and have a 90-day plan, your data expires on May 30th. Any unused data is forfeited after expiration, so plan accordingly.
Check your Ryoko account anytime to see your exact expiration date.