How often should a landlord redecorate?

No, as long as the property is habitable, a landlord never has to "redecorate". And does your mother-in-law actually want to move EVERYTHING out of her apartment, including herself, for the several weeks it would take to redo the place? If she does, and if she wants to pay for new paint, carpet, etc., she could ask the landlord if he or she is willing to have the work done. I would never attempt to paint or carpet while someone is living in the unit - what would be the point?

The law generally only requires that a landlord keep an apartment in habitable condition. In your MIL's case with her being there so long it shouldn't be hard for her to start a dialogue with the landlord to see if they can work something out. Again, he doesn't have to do it but for being there so long it would be a good gesture for him to do so. If he won't and she considers this a deal breaker, it may be time to relocate. If she brings this up that too may help get him to be persuaded...but he doesn't have to.

Never - a landlord provides a livable space - painted up - clean (we hope). I don't think landlords "decorate" unless they're out of their minds.

Unless there are health/safety concerns, Illinois has no law that require a landlord to ever remodel.