Buildbot uses past experience to guestimate the estimated completion time. This is what the manual states:
List of names of metrics that should be used to track the progress of this build, and build ETA's for users. This is generally set in the
If true (the default), then ETAs will be calculated for this step using progress metrics. If the step is known to have unpredictable timing (e.g., an incremental build), then this should be set to false.
The length of time a build takes depends on how many packages need to be rebuilt and how long each of those packages take to build. This varies widely depending on the dependency tree of the packages being rebuilt.
The longest time a build is allowed to continue is set in the buildslave's configuration as 'timeout'; for builder: lsstsw, the value is 4 hours.
Since this number is highly unreliable, I suggest that it be removed from the status display.