The most I've ever spent underground is seventy two hours. This was way back at Easter 1955. I joined three other members of my club (Op Mole) in camping down a lead mine located near to the Jugholes system, from Good Friday to Easter Monday. The reason we did this was because the mine, a pipe working, which was large, with masses of passages (some too tiny for us to explore which indicated the use of children - the earliest records of the mine that I have come across date back to 1535), caverns and mine workings, and we had never fully explored it. A through trip of the system normally took over eight hours before we resurfaced and we only ever saw a fraction of the workings, so this decided our decision to spend our Easter underground. I think it was also partly inspired by Geoff Workman who about that time was spending or had spent a much longer time under ground at the Stump Cross show cave at Pateley Bridge. Whilst two of us made the descent to the two hundred foot level, the other two lowered our gear down the main shaft, we started unloading and had made base camp by the time the other two reached us.
We were able to explore a lot more of the mine than previously and roughly surveyed (to CRG basic survey) where we could. After a day passed we noticed our clothes, sleeping bags etc were getting damp, and one of our members lit a fire expecting the smoke to rise up the shaft, what we didn't realise was that on the surface it was a typical wet, damp, cloudy Derbyshire day, consequently instead of the smoke rising, it was pushed back down the shaft and consequently filled the mine instead, so we didn't get much exploration carried out that day until the smoke cleared (a bit like the Red Soil mine murders at Magpie mine in 1833 when the straw was lit below ground, we couldn't help but think about that). We also found our body clocks changing staying awake later and going to bed about 4 a.m. in the morning. However, the highlight of the trip was to be awoken late on Easter Sunday by a cheery whistling and singing, and a faint light bobbing around in the passage by which we'd entered the mine; no it wasn't a "knocker", a "boggart" or a "little green man" but a mad caving friend of ours from Nottingham by name of Frank Sheen (sadly he's now deceased) who knew of our plans and who decided to pay us a suprise visit. Alas for him, half way down to our base camp his carbide lamp had played up, he couldn't get it to work, so fixed a candle onto his helmet (Bevan Boy type) and instead of turning back, he'd carried on and hoped to his good luck that he would encounter us, fortunately for him he did.
To cut a story short, it was most certainly an experience, which we never repeated. I suppose it was an endurance test really as none of us had the modern equipment or caving clothes that are around today, they hadn't been invented. When I see my son's present day caving gear and try to compare it to what we wore and used, there is just no comparison.
I'm sure that there are people on the forum who can far surpass our underground trip, let's hear of your experiences and adventures. My ex No.2 was on the second bottoming party of the Eldon C.C. expedition to the Gouffre Berger in 1962, which I think spent about the same time underground but of course that was purely a caving trip.