Most eyelash extensions are artificial. They are made of a special kind of polyester, with a tapering tip and a broader base for fixing it with glue to your natural eyelash. The basic model is called synthetic. You'll also hear about mink and silk eyelashes. (You can actually get mink eyelashes – but they are expensive.) But generally, descriptions like 'mink' and 'silk' are just marketing names for a specific type of synthetic eyelash. The 'silk' eyelash is normally quite dark and glossy, and generally good for dramatic and glamour looks. The 'mink' eyelash is more matte and feathery-looking, and generally good for events like weddings, where the bride wants to look beautiful, but natural, in the photos.
To start with, these are not mink eyelashes, but mink fur adapted for eyelash extensions. Some suppliers say it's fur collected after it has falled off. Some say it is fur which has been combed. I'm not sure combing wild mink is terribly kind. You'll have to make your own mind up. But if you eat chicken, you're probably already making some kind of compromise on mankind's inhumanity to animals.
The glue is designed to be used near the eyes. Some people can have allergic reactions – redness, irritation etc – from ingredients in some kinds of glue. So your salon will ask you to take a patch test 24 hours before your appointment to see if your skin shows any allergic reaction to the glue before it is used on your lashes. Your eyes are shut the whole time while the extensions are being applied, so the risk of glue getting into your eye should be small in any event.
If you have a full set – basically, one extension to every one of your mature eyelashes – it could take around two hours. For a half set - where it's one extension to every other eyelash, or mainly just on the outer edges of the eyelid - then it might be in the region of an hour. Your regular infills, won't take so long obviously.
The extensions themselves, could probably last a long time. But the limiting factor is the natural shedding of your own eyelashes. You may not be aware of it, but your eyelashes are constantly growing, falling out and been replaced. That's why they don't grow as long as the hair on your head. So gaps will begin to appear as your own eyelashes are shed naturally. For that reason, you need to go for infills every 2 to 3 weeks, and then you can maintain the look year round.
Appropriate eyelash extensions, properly applied, don't damage your natural eyelashes. By appropriate I mean one's which aren't too long or too heavy for your eyelash type. Your natural eyelashes can carry eyelashes extensions which are up to a third longer, but that's about all. Also, some eyelash extensions are heavier and wider than others, and they can only be put on thicker eyelash hair. If you have very thin or fine eyelash hair, you are probably confined to the finest of eyelash extension hairs.
Having lash extension should be completely painless. You may even fall asleep. If you are one of the rare people who have an allergic reaction to glue, then you might experience some irritation, but nothing serious and you can get eye-drops for it. But you should have a patch test before the procedure to check for this, so it shouldn't ever be a problem.
You probably need them every 2 to 3 weeks if you don't want gaps to become visible. It's up to you.
You definitely mustn't let your eyelashes come into contact with water during the first 24 hours after you eyelash extensions or any touchups/infills. It takes that long for the glue to fix fully. After that, you can wash, shower, and swim normally.
You shouldn't really need make-up or mascara. The lash extensions will contain the colour. If you do add anything, do it very lightly and on the ends only. Do not use anything oil-based, or any remover which is a oil-based, because that will weaken the glue and may cause the extensions to fall off.
Theoretically you can. But bottom lashes are so short that any extensions would have to be limited to 3mm or 5mm at the most, or the natural lashes wouldn't be able to support them for long. But unless your lower lashes are particularly thick, you probably can't have extensions on your bottom lashes.