How often is it okay/are you supposed to, check your hen's nest box? and is it okay to check if she is inside?

It would depend on whether you have eggs, chicks etc. If you have a pair down to breed I usually check ever 2 days, to note dates of eggs so you know when they are due to hatch. Once the chicks hatch I still check each day to make sure chicks are fed etc. It's a personal preference I think, mine are caged when breeding and used to me opening nest box so it's not a problem.

As soon as I give a pair a box, I always tap on the side of the box and open it for a quick peek twice every day, morning and night. This gets them used to my routine, so that when there are chicks to check on, the parents already know that I will be poking around at those times.

 

It's okay if the hen stays in the box. The tapping is to give her a warning, and usually they will fly out before I get the door open. I open the door slowly, in case they are still in there, and I make sure they don't fly out towards me instead of the cage. It's a good idea to shoo the hen out of the box anyway, because one of the things you want to be checking for is that the hen isn't egg bound.

 

I like to make sure she can fly and move normally, and while she's out, I make sure she looks healthy, and check for signs of egg bum, if an egg is due.

 

Other people on here have mentioned having hens who defend their chicks or eggs, and won't leave the box, and who bite like heck. The only one I ever had that did that was actually quite gentle to my hand. I think she was making a token effort at defence.