It's rather variable for me. Sometimes just for the duration of a session, be that session masturbation or sex with my partner. And the length of time that a session may last could, and has, extended to more than several hours.
I have never heard of a tunnel plug. Wouldn't letting air in be a bad thing??
No. The intestines move via peristaltic activity and pretty well "collapse" down when empty. I would never make the open port too large, as I guess, theoretically, you could dry out the mucous membrane's directly inside the rectum while wearing a plug like this, however, with a small tunnel (hole) to allow the venting of gasses, the plug remains very comfortable to wear and the fact you have to use a thick anal lube (and reapply it about every hour) to the plug probably makes the membrane problem a "non issue". At least I have never had any problem with it.
Does anyone worry about anus/rectum getting and staying stretched somewhat after using week after week and month after month? I noticed it's a lot easier to get a larger (1.5 diameter +) plug in now than a couple months ago.
My long term plugs are always very, very slender. In fact, all my anal toys are very, very slender. I am not a big one for large plugs at all, so I know it has never been an issue for me. In fact, despite being an anal play fan, I am a "tight ass", literally. During my routine colonoscopy, the doctor asked if I had trouble with bowel movements, since I am "so tight" he could barely get the scope in without really working at it. I didn't fill him in further on the things and people who frequently visit where he was having trouble going!!!