I think it usually lasts 5-7 days easily enough (maybe longer if you don’t use it with sensors). My daughter has let hers go dead a couple of times though, once at school and she had to go to the medical room and borrow someone’s phone charger and stay there until it had charged up enough to last the rest of the day! Can you imagine that conversation, I think it actually went something like this: “so you are attached to this machine which keeps you alive, and you didn’t charge it up and now it isn’t working?” Daughter - “er, yes” 🙄 Teacher - “you’re an idiot!”
Thankfully she has learned from her mistake and now carries a battery pack and cable in her school bag all the time just in case she forgets again!
Tandem recommend that you stick it on charge for a top-up for 10-15 minutes a day while you are in the bath or shower because you have to take it off then anyway, that would also solve this problem! I’ve never managed to get my daughter into that habit either!