Normally twice a year at continuous use. But in many cases less than once a year. It´s like answering "How long will a light-bulb last?." The sensor is not used up in the same sense as a light bulb but it ages and may become insensitive over time. It all depends on how much you use it.
a) when you are looking for leaks smaller than what pressure decay can find. b) when you have variations in temperature in your test objects c) when you have soft objects d) where you risk having leaking connections e) when you also want to locate your leaks
For certain applications you can speed up the process by using a bit of plastic foam, 5mm thick and of open cell type. If you apply the foam around for example a tube joint, it is often enough just sticking the probe into the foam at one point and get a gas indication even if the leak is on the other side of the tube. This is of course dependant on the dimension of the tube in question. You will have to try this specifically for your case. The reason why this happens is that the foam does not hinder but actually protects the gas from being vented away by the normal movements (draft) of the air. The gas seeps throughout the foam which will give you a gas indication whereafter you can remove the foam and start locating the leak. The foam actually gives you a fast and early indication if it is worth while to search the entire joint. Try it out and please let us know what you think about it.
