What is the least amount of grout space for 6x24 inch tile?

Normally 1/8"

1/8th inch

So when it comes to choosing grout lines, there are a few guidelines, but it really comes down to what you like from a design perspective.

As a general rule, floor tiles should have grout lines of 1/8 inch or larger. Some really large-format tiles, 18 by 18 inches square or larger, look good with very narrow grout lines that are filled with a grout colored to match the tile. This technique produces a floor that looks like a single slab.

Wall tiles, such as the 4-inch by 4-inch style commonly used on bathroom and kitchen walls, often are self-spacing. They have small protrusions called lugs that fit against adjoining tiles and create an automatic space for grout, normally 1/32 to 1/16 inch wide. Grout lines on walls usually do not exceed 1/16 inch because the tiles need to be stacked to help support them during installation.

A small grout line width, about 1/16 inch, is possible if the flooring tile is consistent in size (also called a rectified tile as mentioned above). Installing rectified tile flooring with small grout lines decreases maintenance requirements and looks great; however, it may be more difficult to achieve as there is little “wiggle room” during installation. Unsanded grout is recommended for small joints up to 1/8 inch wide

Medium-sized grout line widths — 1/8 to 1/4 inch — require the use of sanded grout and are typically featured when moderate size variation occurs among the tiles or when tiles with lugs are used. Because a medium size grout line is easier to work with, and in most cases can hide some slight imperfections, it is often the choice of both experts and the average DIY homeowner.

Tiles that vary in size and shape need large grout joint widths — 3/8 to 1/2 inch. Large grout joints will require the use of sanded grout. Marble flooring installed in different-sized, repeating patterns, such as the Versailles pattern, require large grout line widths

At least 1/8". But depends if on floor or wall. Floor grout is thicker than wall grout. Make sure if laying on floor it's over 3/4" plywood under 1/2" concrete board or it will crack.

Dee, you buy a bag of blue tile spacers at the hardware or floor store (dept.). They look like x-es because you place them at the corners where you place the tiles. And that’s the minimum recommended spacing.

1/16

You can buy Tile Spacers and that will sort the problem for you with out the need to measure all the time.