How far off the baseline can a runner be if there is no play on him?

Answer 1

No, he cannot. The only time he can leave the 3 foot baseline is to avoid colliding with a fielder who is attempting to field the ball.

This rule is also supposed to include runners who purposely slide out of the baseline to break up a double-play, but sometimes Umpires forget how short of a distance 3 feet is.

Here is the official rule:

Rule 7.08 (1)

A runner is out when he runs more than three feet away from his base path to avoid being tagged unless his action is to avoid interference with a fielder fielding a batted ball. A runner's base path is established when the tag attempt occurs and is a straight line from the runner to the base he is attempting to reach safely.

This rule also helps to explain why it is okay for a runner rounding a base to swing more than 3 feet out of the baseline as they turn, because there is no tag attempt being made at the time they are out of the 3 foot boundary.

If, for any reason, the batter is already out of the baseline before a tag attempt is made, they are not allowed to Dodge the tag attempt by moving back to the original baseline. Where they are currently at when a tag attempt begins is where the new baseline is established, and they cannot run 3 feet out of where they currently are. They must attempt a 3 foot wide straight line from their current position to the base they are trying to reach.

Answer 2

In Major League Baseball, there is no limit, this is why you see some players taking very wide turns. However, if he goes into the dugout or something like that, he could be called out.