The instructions say that it should last for around 3 months and takes a couple of weeks to get going once you have activated the powder in the trap with water.
At the time of taking this photograph, mine had been in action for around 10 days and there must be over 5,000 flies in there.
As you can see in the picture, the mixture is at the bottom, the black part is all the flies that it has already caught, with more about to join the pile once they are fed up with flying around at the top trying to get out, they then collapse exhausted and fall to join their dead mates.
Mind you, it doesn't half smell, so you need to find a site around 10-20 metres away from where you are bothered by them, don't worry they will find it wherever you put it. Hang it up to do its stuff and at the end of summer, if the bags not full to over flowing with flies before then, put the dead flies into the compost bin.
Check out the video, below and look at the crazy flies.
You can get the Red Top Fly Catcher off eBay on the following link;
http://shop.ebay.co.uk/i.html?_from=R40&_trksid=p5197.m570.l1313&_nkw=red+top+fly+catcher&_sacat=See-All-Categories