I have used Castor Oil for years on my eyelashes and for me, it makes a huge difference in my growth and thickness. I don't know if it would really make existing eyelashes longer (maybe?), but I have trouble at certain times of the year with my eyelashes falling out (and even some "bald" spots on one of my eyes...always the same eye!). I keep an eyedropper by my bed full of it and just drop a few drops onto my fingertips and dot it across my eyelashes and gently rub it in each night as I get into bed. I can tell when I'm using it and when I've forgotten to use it for awhile as my eyelashes are shorter and much sparser. I can honestly see a difference almost immediately when I start using it again (after forgetting about it for a bit).
I use this brand and love it (and a bottle last forever it seems!): https://www.amazon.com/gp/product/B00A2F4PIQ/ref=oh_aui_search_detailpage?ie=UTF8&th=1