For comparison's sake, let me share what the showering process and time was like for a 450-ish pound, five foot tall woman with severe lymphedema in both legs. She was much older than Amber and lacked even the level of mobility Amber's got but she was at least 100 pounds lighter.
--The bathroom had a shower/tub combo common to most 1980s-built suburban homes. The tub had been converted to a walk-in tub
using a method like this, so the woman could get into the shower without lifting her legs. Getting into the shower and properly situated took several minutes. It would have been far easier if the tub bad been a genuine walk-in tub with a door that sealed but the super morbidly obese seldom have money for that sort of thing.
--There was a shower bench that she on sat facing out toward the bathroom instead of forward facing the shower faucets because if she sat normally water got everywhere, even using a hand-held shower nozzle, and her legs could not fit into the shower properly. The open side was part of the water-everywhere problem but if she sat normally there was no way to rinse the right side of her body without essentially flooding the bathroom even worse. So she sat with one leg nestled in the step opening and the other leg sort of stretched out to the side. Rinsing her in a manner that avoided soaking the floors and toilet took careful manipulation that took much longer than you would imagine because once on the bench, she would have to stand with her left foot in the opening of the bathtub and her right leg stretched and hip still sort of in a sitting down position. It's hard to explain the logistics but just bear in mind she had a lot of trouble with moving and balance even with a bench.
--Even with a hand-held shower head, she had a lot of difficulty washing her hair. Her arms were very weak, so someone else had to lather her hair and she could generally rinse it. That took several minutes.
--Here's where the biggest time suck came into play. The actual wash part of the shower was pretty rough. The amount of weight on her stomach and hips made it impossible for her to bend over, even when seated, to clean her legs, and her skin was too compromised to use a regular telescoping rag on a stick device that enables many fat people to clean themselves. She could have used one of them, and she did use one on her upper body and back, sort of, but the rolls on her legs had such weak skin that she could end up seriously abraiding skin covered with intertrigo. She needed someone to wash and thoroughly rinse the folds she could not reach with a very soft bath sponge or wash cloth. She also needed lots of perineal hygiene assistance, which means someone had to wash the areas around her labia majora and her butt. That was a massive undertaking because she would have to partially stand and it could take a while to get her clean if she'd had a bad toilet day. The process of her washing her face and as much of her upper body as she could, and what went into getting her lower half cleaned without fucking up her very yeasty, seeping rolls, could take up to 15 minutes. Longer if she was tired or was experiencing more pain than usual.
If I don't factor in all that it took to get her into the shower or what needed to happen when she was out, but I do include the time it took to wash her hair, those showers took about twenty five minutes minimum. But from beginning to end - undressing for the shower, getting set up on the bench, and all the washing, followed by thorough drying and powdering, could take close to an hour. It would take longer if she was tired, or needed rest breaks.
If Becky can somehow help Amber wash in the boys' walk-in shower, and if Amber isn't washing or conditioning her hair, there's a chance it could take ten minutes, especially if she is not taking the time to engage in scrupulous perineal care and making sure every roll is cleaned and rinsed thorough.
If it's just Amber crammed into that tiny shower in her and Becky's bathroom, it's still possible if she's just wiping a wash cloth here and there and not washing her hair. I guess with Amber it comes down to semantics, by what she means by a shower. We all mean the process in which a single human being thoroughly washes and then rinses off their body, possibly washing their hair, possibly shaving legs or armpits. Amber may be saying she showers in ten minutes without sharing that Becky helps her, or that she can do little more than let water fall over her rolls before she can't stand up any longer.
Also, it's estimated that the average woman in the USA spends nine and a half minutes daily in the shower. These average women can stand up for hours, wash their own hair and bend around easily to wash and shave. Even with 30 extra seconds it is almost in the realm of science fiction to think a 600 pound woman whose legs have more folds than an arrangement of origami swans could accomplish such a feat.