I think different people have nails set at different angles - with me they stay on well enough provided there's as much nail on my right hand as on my left, fretting hand. I think that's probably rather less than would count as "reasonable" length. The metal ones stay on better, because you can mould them better to the finger (or thumb).

I'd advise anyone who has never been able to feel comfortable with normal fingerpicks, but who would like to use picks, to have a go with these.