No. Sparks never expire.
Buy a pack today, use a few, leave the rest sitting for six months, and they'll still be there when you come back. Buy another pack later, the new ones stack on top of the old ones. There's no decay, no monthly reset, no "use them or lose them" pressure.
This is intentional. There is no clock to beat. Sparks you have bought sit in your account until you choose to use them.
The one edge case: if you delete your account, your Sparks are gone with it (we can't move them to another account, and we don't refund unused Sparks on deletion). If you just cancel your subscription, Sparks stay. If a Premium subscription lapses, your Sparks remain usable for Ask under the current lapsed-mode rules. Your statutory consumer rights always apply.