Hello, what should the invitation letter include if I'm requesting for F-visa

The accommodation period depends on how long you are allowed to stay in China with your F type one. Generally speaking, a F type one is no longer than six months. By the way, the invitation letter for F type one is issued by duly authorized unit. If the company that invites you can't issue it to you, he/she must apply for it from the local Foreign Affairs Bureau. It's a bit complicated.