Mr. Moto: Do you have a cite for these warnings? I doubt whether a block of dry ice could sublimate enough CO2 to create a health hazard in the volume of an ordinary sized house. Especially if kept in a freezer. And if it is true, then all that the OP is planning (and that I’m recommending below) is a serious suffocation risk.
Anyway, as to the OP, you haven’t said what effect you want to create. If you want to fill a room with a low-lying fog, everything Nametag has said is correct. If not kept constantly “on the boil,” your fog will disperse relatively quickly, especially if people are walking around.
Many years ago, on a stage show I worked on, we chopped about five pounds into small chunks, dropped them into a trash can a quarter-filled with hot water, and put on the lid, which had a hole to which we taped a length of four-inch flexible duct. We were able to fill a stage about 100x50 feet for about ten minutes. Use work gloves to handle the dry ice and chop it up. Once the water cools off, you get less fog. But there may still be some solid dry ice in there that will produce more fog if you add more hot water.
At only $1 a pound, why not conduct an experiment to test your equipment and techniques and see how long the fog will last in your exact circumstances?
Another (simpler and less expensive) option is to place small chunks of dry ice in the bowl of punch or cider to create a spooky bubbling concoction. Of course, be VERY careful not to consume the dry ice!!! (Interesting side note: the dry ice will tend to carbonate the drink after a while.)
You may want to look into renting or buying a fog or smoke making machine from a party or theatrical supply store. They burn a non-toxic oil and will probably produce the effect you want for a longer time with less trouble than the dry ice method. (If you can get one at this late date.)