As soon as you think they need it really,some mums are great "housekeepers" and some just don't do mess,start with something easily accessible for them and try and watch them and as and when of mum isn't helping you step in,sometimes kitts are naturals as it were and sometimes as with potty training human babies they need reminding/training-it's great when they learn-ya like Aaah your such a clever kitt and ya get well excited-i've said this before but if ya human babie dumped purposefully on ya rug-you'd be thinking "Ya little sh*te ya" if it's your fur kitt ya like "Aah bless-let me show you where ya should go darlin'"