i never have an empty tank unless its in need of rebuilding/re-wicking. if the wick and coil are good to vape, the tank is always full, or semi-full. i pair many of my vapes with various beverages throughout the day so i like to have a complimentary vape ready to go when i change beverages. that and i just like to switch up my vape constantly.
also, i have a rig i keep in my bike bag attached to my bike. i keep it in there year-round so i dont forget a vape when i go riding around. at the end of the nice weather season, i leave the vape there still and its always got a full tank. my point is, i leave juice in the tank for months at a time without any problem. the first vape or two after the winter are the warm up vapes and dont perform well, but after a couple toots its rockin and rollin.