UKBA will send you an email or letter when they have received and opened the application pack. This is when the 12 day expected processing period will begin.
There have been a lot of issues with UKBA recently due to budget cuts, staffing cuts, strikes, poor training, etc so processing times have been longer than usual, at least domestically. I don't know how things have been going overseas. I would say that you should estimate that they will exceed the typical acceptable service standard by about 50-75%, just to be safe. I recently received my Tier 2 work visa (moved over from a Tier 1 PSW) and it took a lot longer than their estimate. The acceptable service standard is 4 weeks from the biometrics appointment (which is at least 3 weeks after you submit your application) but UKBA took just over 7 weeks from my biometrics appointment to approve and send my residence permit. Good luck!