I have used the castor oil to regrow eyelashes with great success but you have to be patient.
I was on another beauty forum and almost everyone who tried it had success. I used the cold pressed castor oil which I purchased at my local health food store but there were others who just purchased the processed castor oil from the drug store and it worked as well. Castor oil is known as a laxitive and you would find it in that section.
I would take a q-tip brush it on the base of the lashes and massage it in. It does'nt irritate the eyes. Right away you will notice with what lashes you have that they are darkened. (Before mascara women would use castor oil for this purpose)However, the downside is that it takes some time. Some reported noticable regrowth in four weeks, for me it was closer to 6 weeks, but I now have a full lash line. It is probably better to do this at bedtime as the oil can be messy. I would think that you could do this as often as you wanted to.