Worming, how often?

[frickenchicken1]

I live in Australia and we have a levamisole wormer which I bought because it was on clearance at my pet store. I've read online that chickens should be wormed every 3 months, some say every 6 months or yearly and then others also say to only worm when they have worms because they believe that the worms can begin to go immune to the worming medicine due to routine worming. What do you all believe? Also, is levamisole a good wormer, who has had experience using it and how has it worked for you? My birds do not show signs of having worms

 

[i like emus]

I have had tons of chickens and never wormed them so I really wouldn't worm them unless they have worms that's my expierce but do it if you think you have to