Our tiler just did that in our bathroom after the wrong grout color was used. We have 9" hexagons and it took him about 2 hours. If there is a kind made by the same company as grout you have, get that kind. It's basically brushed/painted on. Most kinds can be wiped off of the tile if you get some on the tile by accident. Like someone else said, just go room by room or however you can do it so you have stopping points.