It should be somewhere between moist and soaked through. It should be wet enough to have a darker shade to it than usual but not wet enough so that it's slippery and dangerous.
It should be done every morning and if it completely dries out in a few hours it might be a good idea to wet it down again.
EDIT: I like putting my thumb over the hose's nozzle because I'm lazy like that, but this tends to take slightly longer. It helps get the right mix between moist and muddy I've found, though