The rewards gift certificates do have an expiration date and although I can't recall now how long they last, I believe it is approximately 6 months. You can use the certificates with other promotions. For instance I had $250 in certificates and I used them on Friday for the private sale. My purchase qualified me for another $500 which I will receive in about 30 days. Despite using the certificates the full retail price of my item was counted (not the price after subtracting the certificates). So, in order to get the best bang for my buck, I usually use my certificates in conjunction with another Bloomies promotion.
In order to keep track of your points and certificates I would recommend making an account on Bloomingdales website and linking your loyalist account to it. This way you can keep track of the expiration dates and points overall.
Also you do not need the physical certificate to redeem the rewards. The sales associate can look up your rewards and apply them in one transaction.