Not at all, if it is done correctly it should be completely painless. Most of my clients end up falling asleep :)
Depending on your set, it should take anywhere from 1 hour for a classic set to 3 hours for a full volume set. Fills usually take 1-2 hours.
My sets usually last approximately 3-4 weeks.
Just like the hair on our heads, the eyelashes on our eyelids grow. After about 3-4 weeks you'll notice that some of your lashes have grown out, fallen out, and need to be replaced. During your fill, your natural lashes and extensions are cleaned and unsalvageable extensions are removed and then reapplied.
Just like with any beauty service we have done to ourselves, it puts a little bit of wear and tear on whatever that may be (eg. dying our hair or doing our nails).
It really depends on each individual client and the state of their natural lashes. Some people have strong, thick and full lashes and don't require breaks, but others have finer, thinner and sparser hair requiring annual breaks.