When I smoked I would always get another pack of tobacco when I was down to the last 'chunk' in the packet. That last chunk was probably about 15-20 rollups worth, but I'd get 'The Panic' about getting more before the shops shut.
I appreciate you don't get 'The Panic' with ecig liquid as you've probably got a stockpile like I do - about 10 different bottles in a little bespoke tupperware box.
However right now as I'm on holiday, all the liquids are in my bedroom where my daughter and boyfriend are sleeping and I'm down to the last ml in my ecig... 'The Panic' has returned....