Do Spendable Merits expire? Or they never expire.
As stated above, currently it does not expire, although things are not immutable, and since it was hinted that they may decade at some point, that could happen at some point down the road.
In any case, they would not affect recently generated sMerits (as yours are), and would have more chances of starting in any case by affecting unspent airdrop sMerits - those generated when the Merit System kicked-off, and that were assigned in accordance to the following formula (totalling 600k sMerits forum wide at the time):
<…> The exact formula was:
yearActivity = activity in the last ~year, max 378
freeMerit = the merit (not sMerit) you started with
modifier =
0.1 if member
0.2 if full member
0.25 if sr member
0.35 if hero
0.4 if legendary
return (yearActivity/378) * modifier * freeMerit
So the most you could get is if you were a Legendary who maxed out your activity points in the last year, in which case you would get 1 * 0.4 * 1000 = 400 sMerit.
Note that I later made an adjustment giving 500 extra merit to some hero members, but no extra sMerit was given here.