How long should a draft round last by official rules?
Appendix B of the Tournament Rules provides required and suggested time limits for official events. Check this document for all the details you need.

In general:


Required minimum game time is 40 minutes. Recommended game time for typical Limited/Constructed Swiss rounds is 50 minutes.
WotC recommends giving players 30 minutes to register and construct a draft deck.
The individual booster draft timer values in judge apps come from a chart in Appendix B (page 43 as of the current revision). Note the sentence after the chart that provides short "evaluation" periods in between packs as well.


Note that organizers are allowed to deviate from most of these conditions, but are required to announce it clearly in advance.