Ideally administrators should aim to automate everything with scripts and then document the use of the scripts. That way, if you're hit by a bus the whole system is already in a good shape for others to continue. Personally I think it's part of the administrators responsibility to keep the system in such a good shape. Management should also understand the importance of this and allocate enough time to keep systems in good shape. Doing things like that as an afterthought takes a really long time and makes knowledge transfers really painful.
If your management has allocated only one day for the knowledge transfer, they're taking a huge risk. There's no way you can teach everything about a system in a day. What you can do, is tell your current management realistically about the situation and the risks. After you've gone through the system with the replacement talk to the management about how the knowledge transfer went and make a signed contract about how much support you are willing to give afterwards and how much it will cost them.
With a contract it's up to the management to decide on how much their system is worth to them and you'll get compensation on doing extra work. Who knows, maybe the management will realise that spending a week on the knowledge transfer upfront might be cheaper than paying you afterwards.