I've made a 10 point guide so you know when to change your bedding.
1. Pat the sheet, if the dust plume rises over 1ft time to change.
2. If you hear an audible crack when you sit on it breaking the crust, time to change.
3. Place an unused (important) sheet of toilet paper flat on to the bedsheet. Lift after 1 minute, if the sheet tears due to soaking up variolous liquids, time to change.
4. If you notice a colour change in one section that is larger than a football, time to change.
5. Every time you change your bed. (This my be specific to the homeless, but home owner should probably do it as well)
6. Every time you turn you mattress over. Bedding works best on the side you sleep on.
7. Every time you get a new partner, you want to appear clean at the start.
8. If you see movement when looking at it, and you don't have a optical illusion design duvet, change it or hover the critters off.
9. If it starts to act like fly paper it's a sure sign to change.
10. If you find any dead bodies in it, best to change.
Follow this guide you should be up to the basic standard.