My wife and I get them each time we enter/leave BKK and fly PG as Premier members of their FFP. For the last year or so we've rarely had a need to use them. I also get them from SQ and UA from time to time. When we do use them we use the oldest one and never hear a peep from anyone in the fasttrack area. We've built up a stash at this point we'll never get through all of them!

Bottom line: So long as you have an orange CIP Fasttrack coupon you can use them. Date, airline etc doesn't seem to matter. YMMV.