Thai SIM card credit does not expire, as long as you keep your SIM card active.
To keep your SIM active you must make one call, SMS or data transaction in 6 months, and/or a minimum 10 Baht monthly topup. Topping up 50 baht every 2 months is usually enough to keep the SIM active, but this depends on the type of SIM card. Tourist SIM cards are designed to expire quicker.
You can check the date of your SIM expiry at any time buy using the corresponding USSD code.
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