The "Last build" column in the Data Sources -> Indices tab of the dashboard reflects the time that has elapsed since the last updatedAt timestamp for any indexing/build operation in the corresponding index.
You can see an example response that includes this parameter here. This response indicates that the operation has been taken into account, but it may not have been executed yet.
As such, the "Last build" column reflects the timestamp that the operation was taken into account by Algolia but does not account for the duration of the time to execute the indexing task.