On page 123 for mechanics skills, it says it requires "appropriate tests and times" and refers you to page 125. On page 125 of the BBB, under "Using Technical Skills to Build or Repair", it says to use the thresholds and intervals as listed on the table on the same page. The table lists thresholds and dice pool modifiers, but no intervals, unless I'm missing something.

Soooooo... What should the intervals be? Hours? Days? Minutes/Hours/Days/Weeks going down the threshold lines?

I'd use the table on p. 58 in the sidebar.

I'd also cross reference against the table on P. 303, Availability Interval, and use that to set the threshold. For example, if repairing something, you can assume it might take more or less time then simply buying a replacement, but it will give you a scale to go against.