No its not that hard, There are some VERY good guides on youtube for it.
You just have to make sure you do Everything right each time, and make sure you make an order of where each and every screws place is.
They are all different sizes.
I can do an iPhone screen in under 20 mins now but it has taken me years to become that fast with no errors
You would probably be able to get your first done within a few hours or if you learn fast then under an hour.
You just need patiance
Something i have learnt though.. The people who work at the apple store and who are called Technitions actually have NO IDEA of how to fix an iphone, most useless buggers i have ever met.
I was turned down a Job as an iPhone tech due to lack of experience, even though my CV showed that i have been doing it for years -.-