I haven't completely changed litter in about 18 months.
Now, before anyone goes ewww yucky yucky nasty, let me explain.
I follow this method of litterbox maintenance. It involves scooping at least twice a day, including cleaning/disinfecting any stuck-on residue with a bleach solution. It works great for us. The litter actually stays cleaner than it did before, when I changed it at least once a month. It's not for everybody, as it requires more time each day to maintain, and the self-discipline to keep up with it. But I much prefer doing thorough daily maintenance/cleaning than having to change out all that litter once a month or more.