I have looked on several manufacturer sites and it seems that most of them hypothesize that they should last about a month. HOWEVER, they are not manufactured with NASA-like precision and I would consider a month to be an average value.
In reality you are probably safe anticipating a useful life of a week to a couple of months depending on how much you
vape. There are outliers that will die sooner and those that will last much longer.
Some people advocate cleaning them and others believe they should only be blown out regularly. You will need to decide what works best for you. (Personally I clean them in vodka fairly often.)
You will want to replace them if they stop performing well even if they still 'work.' If this happens you might consider some of the more aggressive cleaning techniques, but be prepared that the atty might not recover.
In any case, make sure you have spares! Periodically shipments from overseas get bogged down and the suppliers might be out of them for a while.
