You mean the parts? The ecig you hardly use would last the longest.
Batteries are only good for somewhere between 150-250 charges unless you kill them first by buying unsealed or auto batteries and getting liquid leaked into them, or if you torture batteries by putting stuff that is too low ohms on them. I have worn out (by charging) the original batteries of every model I use daily.
Atomizers - IMO they sucked after 6 weeks of use no matter how I cleaned them. So I bought new ones and I only use them at home for testing juices and occasionally dripping juices I use rarely. I use cartomizers for everyday vaping. Using thin clear juices I can run 15-20 ml through a carto before it feels degraded enough that I feel I deserve a new one. For me that is about 7-10 days per carto (since I alternate vaping 2 models, each with a carto, that is 2-3 weeks for the pair). My cleanest clearest juice gives me the longer lasting carto, sometimes over a month, using it 1/2 the time. I am very good at keeping my cartos damp so they don't singe and ruin the flavor.