Pre-Mlife, Borgata comp$ expired in 6 months if not used. [When you logged onto your acct at www.theborgata.com and clicked "comp$" you could see a table (by month) of expiring comp$.]
I believe that under MLIFE (even at Borgata) comp$ NEVER expire as long as your acct shows activity for 6 month/12 months depending on status--is my belief correct?...NO official statement is noted on www.theborgata.com.
PS: There were times when I had to buy (super)overpriced wine at the wine store AND use comps at 2:1 in addition, just so I wouldn't lose my comp$.