No rule. But that's not DLP anyway.

DLP is moving out into the traffic lane when there is no faster same-direction traffic present, recognizing that reasonable people can disagree on how close approaching same-direction traffic has to be to be considered "present".

Plus no rule says so explicitly. However, that it's normal to operate in the main traffic lane when faster traffic is not "present" (subject to the above subjective definition) is implied by:

• The rule that says that all vehicles, including slow vehicles, should normally be driven in the traffic lanes (for visibility and predictability).
• Pulling out of the traffic lanes is an exception for drivers of slow vehicles, particular when safe and reasonable, to allow faster traffic to pass.