Im not doc john but I do use invisible.

you will always see you.

others wont see you.

there is an icon down next to the report button.
it shows green or red or gray icon near the report button.

the colors.....
green means others can see you,
red means others know you are not online
gray means you are in invisible mode and members will never know when you are online and when you are not online.

there is no time limit.
it will stay invisible until you change yourself back to not being invisible (let others see you are online)

you can change back and forth at will. no time limit in between.

you can still read and post when you have set your profile at invisible.

all it does is make it so that others do not know when you are online or not. you wont show up to anyone but yourself, in the who's online listing at the bottom of the page

not sure if you show up in the chat list while your profile is set at invisible as I rarely use chat and never looked.

thats the jist of what invisible is when Im using it.