You can put off the inevitable by running a flame through the screen now and then. I remove the tubing and use it like a chillum. Suck VERY gently, though- the smoke is HOT. If your lungs are sensitive, get a friend to do it- he'll really get his head!

As to when, remove the plastic tube and check if you can see through the screen. If not, try flaming it a few times. Usually that will clear it up, but eventually the screen is just too crudded up and has to be changed.

Make sure you have the right sized screen! Talk about frustration! I finally ended up using wire cutters to trim one down! I use a bamboo skewer to put them in. It's a hassle, no matter how you do it! I try to foist the job off on the hubby- last time I traded him a load of dishwashing to do it. (I hate washing dishes, but it was worth it!)- Granny:hippy: