Answer
Do not replace tarantula substrate on a fixed schedule. Change it when contamination, persistent mould, pests, structural failure or a necessary rehouse gives you a real reason.
Routine cleaning
Remove accessible boluses, uneaten prey and obvious waste. Rinse the water dish. Healthy webbing and a stable burrow can be left alone.
When a full change makes sense
Replace substrate after chemical contamination, a serious mite/pest problem, foul persistent odour, repeated saturation or when the substrate can no longer hold a safe burrow.
When to leave it alone
A spider rearranging substrate, burying the water dish or building an ugly web is not a hygiene problem. Established retreats are valuable structure.
During a necessary change
Prepare the new enclosure or holding cup first, move the spider with a catch cup, then rebuild the enclosure around the species' normal lifestyle.