They can run out sooner than 5 years BUT good news they can also just have a blip where you have a period or a few consecutive ones then it rebuilds up a new goop layer and gets back to work again like it did before.
About this time last year i had 2 periods (proper ones) thought that was it -the mirena had died on me early - and was in the process of getting an surgical appointment to get it replaced (mine is surgically inserted in to a bicornuate uterus and can only be removed and replaced with surgery too - so I was not best pleased or looking forward to the change over) anyhow as I said it was a blip, and the beastie kicked in to working again. Happy Days.
Infact i have hardly had a spotting in phase two since those periods. Which is a better result than what i call phase 1.
Don't be too hasty to have it swapped just yet. I'd give it a month or two more to see if it recovers building up a goop layer enough to stop the lining of the womb shedding as a period. Better that, than go through the removal and reinsertion unnecessarily.
Best of Luck either way.