So, I'll be a voice of dissent and say it really boils down to your soap recipe, the conditions in which it is kept a whole loaf, and the duration of time its expected to be used.
I make a very high coconut oil and shea butter soap. Mine is firm enough to cut after 8 hours, but soft enough to shave, cut into little pieces or carve, etc, a year later. I am constantly cutting up old bars as I experiment with new packaging ideas, etc. Unless I make a boo-boo and soap too cool, mine does not crumble or get too hard to cut with a quality stainless steel chef's knife. Mine stays the texture of extremely firm cheddar cheese for a year or more.
Now, other recipes will have different results, salt bars for example, would likely prove quite difficult to cut later on down the road.
Additionally, while I have never left a loaf whole for more than a day, I do frequently chunk up a loaf into 10 oz hunks, to experiment with at a later date. When I cut open a large chunk of soap say a few weeks or months later, its tacky and sticky on the inside for a few hours until it dries.
I'm sure the soap would loose water weight faster in smaller bars, but it hasn't effected the feel of the product as far as I can tell.
What might occur, given enough time however, is warping. Depending on factors like your loaf size, amount of water in your recipe, etc., your loaf may twist or buckle or warp over time as it loses water weight.
Finally, where you keep this loaf for your associates to use will also play a factor... if it's constantly in a humid environment, like a bathroom that has little to no air circulation versus a wide open space with lots of moving, dry air.
So, my advice would be to try and see. Pick a recipe that you feel works for your needs and desires, make a loaf and then try using it,. Its really the only way to find out and troubleshoot, as, as I've illustrated above, there are just too many variables to have cut and dry answer.
As a side note, I love the idea of self-serve single use soap shavings for guests, I think its very cute, and a clever way to replace other single use hand cleansers like liquid pump soap, etc.
My question however is to the issue of sanitation: If you expect your guests to carve or slice off a chunk of soap for their own personal cleaning needs, won't each of these guests be using the same utensil with an unwashed hand to obtain the sliver of soap?
