Currently, they expire after 10-years. From the McDonough County Voice:

Illinois residents with firearm owner identification cards would have more leeway in renewing them if the General Assembly passes legislation proposed by Rep. Adam Brown, R-Decatur.

As originally written, House Bill 3796 would have eliminated expiration dates on FOID cards, effectively making them valid for as long as the owner lives. FOID cards currently expire after 10 years. However, Brown is considering several amendments to make the bill more palatable to the governor’s office and law enforcement.

More at the link. Instead of not having an expiration date, the bill is being modified to perhaps allow a ‘grace period’ for renewing it without penalty.

From what I understand of the FOID card, it’s relatively cheap and once you have it you can purchase firearms without going through further background checks. Ten-years seems like a reasonable time for that. Of course, not having any required card would be even better.