In my case, sadly it's very possible. From the time of separation in October 2012 until March 2013, STBX was taking them 5 overnights a month, wasn't even asking for more, but at our first mediation (when child support was to be determined) he said he wanted 50/50, and he got it..
But I would imagine if you already have a legal visitation schedule in place, and he's not taking all the time he is supposed to, that would be different.. I think it starts setting a precedence..
As far as documenting, of course I keep a calendar, but I also take tons of pictures. I've even taken them when they are sleeping at night, just to prove where they are sleeping..