Practicality of seventh Jhana?
They're practically possible.
Why about the seventh Jhana? The below answers are for 1st Jhana. So, conditions for the seventh Jhana are even more extreme.
So, one may ask, then how come multiple people(conciseness) live in the seventh Jhana realm? Does nobody live in these realms?
Even though it's very rare, concisenesses are living there. (in these realms)
It's because, their lifetime is very long, as the eons pass some people achieve these Jhanas and go there.
How common is it for lay people and for monks both to attain higher jhanas like sixth, seventh and the eighth ?
No monk or layperson would post their most noble attainments on a website. Even if somebody goes to them and asks personally and secretly, they wouldn't tell.
How easy/hard it is?
It's not common. It's very rare. Among lay people, it's even more rare. My guess among monks is 1 in 100,000 and among lay people 1 in 10,000,000. It is rare because they don't put in enough effort, they don't have the capacity, don't have a suitable background, don't have a suitable teacher, etc. Let alone achieve it most people in the world have never heard of the word Jhana. (more than 90%). Even among those who know about Jhana, like 90% of people are just living their normal lives. Even among people who try to achieve any Jhana, like 90% just give up or don't achieve it. That's why it's rare.
Have you entered any of the Jhana states yourself?
No. I say this because I haven't. If I had, I wouldn't say it like this or be even typing stuff on the internet like this. (Not angry or blaming, but just saying).
If yes, what was your method?
Even though I haven't achieved it yet, I do try. A collection of methods. Anapanasati, Meththa, one from 10 main Kasina.