The 'Time Since Last Run' trigger condition prevents a build from starting when a specified length of time has not past since the project was either last run or last triggered by a specific trigger.
Length of Time
The length of time that should of elapsed since the project was last run or triggered.
Elapsed Since
Determines whether the condition should look at the last time the project was run (manually or by a trigger) or the last time it was triggered by a specific trigger.