Researching the answer to this question was somewhat frustrating as there are so many, many conflicting answers. I have seen answers including; they never grow back, they never grow back the way they were, they grow back within a week/month/3 months. A dermatologist website said that it takes about 90 days for an eyelash to fully grow in and I trust this to be the most reliable source. But logically, think about it. How many times have you had an eyelash in your eye, and had to dig it out? This has happened to me many times over the years, and yet my eyelashes are as thick as ever.
Many sources say that the use of mascara and eyelash curlers can damage eyelashes over time; making them brittle, causing them to break, or cutting them off in the curling process. Many different sources confirm that you should remove your mascara with baby oil, or some kind of eye makeup remover before bed each night. Some people report pulling off their mascara by hand; and chunks of their eyelashes with it.
In summary, eyelashes do grow back within 90 days. If you use mascara, buy a decent brand and make sure to remove it before bed time. Only use an eyelash curler on special occasions it can damage, or even chop the hair if you are not careful.