Not all non-subscription MMO has same policy. There are popular free to play games that their premium currency never expires. Shadowverse crystals and Fire Emblem Heroes Orbs are good example.
General > Crystal purchase:
"Do crystals expire?
No, crystals do not expire."
Source: Head to Shadowverse.com and click FAQ-Contact at the bottom of the page.
Fire Emblem Heroes:
Faq > Orbs
"Do Orbs expire?
No, they will not expire."
Source: Head to Fire-emblem-heroes.com/en/ and at the bottom of the page, click FAQ. Then click "Questions about Fire Emblem Heroes" at the bottom of the FAQ.