Given the forum's begun again, I think it worthwhile to spell out Safe Play.
Yes, the scenario should be discussed and agreed in advance, and not diverged from unless to end the game suddenly. Safety words and signals should be agreed and respected, frequently one for "no further" and another for "release me immediately" - any Dom failing in this respect should be reported to the police for common assault. The entire thing should be worked through from a Health and Safety viewpoint - the UK HSE has a 5-point planning procedure which is fairly exhaustive, so you're both preemptively safe (for example, the Dom has been trained in knotting so they don't accidentally block any of the nerve or blood-supply sensitive points) and have a reaction plan if something were to happen (in these scenarios, the primary release is the Dom - what happens if they fail, have a heart attack, for instance? Can someone check in if there's no response after a given time? A mummified sub should no more rely on a single release mechanism than they would if they were into self-bondage). Have surgical shears to hand if you're the Dom, it'll take you longer to release the sub than it took to secure them, possibly 50% more, if you simply reverse the procedure, time you don't have if they're in trouble or simply want out.
From a physiological viewpoint, don't keep someone mummified for more than 12 hours maximum under any circumstances. In practice, an hour or two is usually enough, and should be the maximum for starters. The blood supply relies on a certain amount of movement to pump the blood around, you don't want to trigger thromboses which can cause heart attacks, strokes or pulmonary embolism. Follow it by six hours of physical activity in light bondage maximum, and only the lightest (no ties, for example) while asleep. The Dom is responsible for the safety and well-being of the sub, and is not in a position to do so if they're off having a pee, making food or drink, or has fallen asleep themselves during the night. That is as fundamental a failure as not respecting safety signals. By all means keep wrist and ankle bands locked on, as long as they're not secured to anything, so the sub can get themselves out of trouble if you are doing something like that - they can find subspace in them even if not connected, it's the symbolism which matters.
In the longer term, other aspects are that the body starts to react to withdrawal of water within 24 hours, and of food once the food in the gut has been drained, within 48 hours. That can cause psychological damage as well as physiological, particularly where there is no chance of movement. Similarly to how the body needs a certain amount of movement for the blood supply, it also needs it for waste evacuation, you don't need a constipated sub. There are some ways to get around these, but if you're not medically trained in them, don't try for yourself. Equally, if the sub has a family history of embolisms, fits, or some less frequent syndromes, don't go there. This should be about forms of love, and love sometimes means saying no.
Ed: omitted at first because it shouldn't arise if safe durations are used, but probably worth mentioning anyway. An immobile position can cause bedsores within 24 hours, from creases in what the body's lying on. Rubber suits can help, but even they have centre seams down the back and around the waist - one or two suppliers produce suits from single pieces of latex, a custom fit where the seams are on the sides should be the absolute minimum. Bedsores can become permanent ulcers and at worst become gangrenous - this should be about non-permanent effects, so again, keep the session length reasonable. /ed
So, prior planning and proper preparation not only prevent piss-poor performance (and no sub deserves that!) but also promote pleasurable play for the partners. Play safe, folks.