There is no reward for you here.
You have merits and opportunity to spend it in 30 days.
If you don't spend it in 30 days you lose your available merits.
So, merits are just a way to show to other members that we are content with their contribution to the forum, nothing more.
Nope. sMerits from "normal" Bitcointalk members doesn't have a sort of expiration date. The 30 days thing only applies to merit sources:
There's only sMerit deadlines for merit sources , but for normal users there currently is not limit/deadline. However, it has been proposed that sMerits should decay to encourage users to spend them, but nothing has been implemented for that yet. Merit sources "pool" of merit refreshes every 30 days, and therefore they are encouraged to use up all of their sMerit within those 30 days.