Buildbot uses past experience to guestimate the estimated completion time. This is what the manual states:
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progressMetrics
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

useProgress
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.
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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.

I agree that it is difficult to produce a useful ETA figure. Instead, it would be better to implement a LogLineObserver that reports the current package being processed, probably via updateSummary() and getCurrentSummary(), which looks like it would require subclassing ShellCommand. Before undertaking that, I'd like the current ShellCommand to get a descriptionSuffix=Property("branches").

Closing as Won't Fix. Buildbot is on maintainance-only status, and the issue of accurate build ETA times will be addressed by decomposing the build (RFC-69).