it also depends on the filter. more powerful filters in a smaller tank with sand should have an intake tube that is farther up than a weaker one in a larger tank.
Depends on the filter and the flow rates but anywhere from about 2 inches with lower flows and gravel to about 4 or 5 inches for higher flows and sand. The filter probably has a suggestion right in the instructions.
Lower intake tubes will pick up mulm closer to the bottom, higher intake tubes will pick up more out of the water column. If you have messy eaters that leave more waste near the bottom you would want the filter intake lower.