I've only ever changed egg flats once. That was with the B. lateralis roaches and the egg flats were over a year old that time. I only change them when they start to fall apart in my hand when I pick them up. The egg flats in my dubia bins seem to last a very long time and I don't see myself changing them anytime soon. If the enclosure is well ventilated they shouldn't decay in 4-6 months.