When can insulation against external walls be done for new construction?
If you pulled a permit for this work, you should not proceed until you have your framing, rough electrical, and rough plumbing inspections signed off. Depending on the work, there may be more inspections you should wait for (i.e. rafter tie-down straps, rough mechanical...) your building permit likely has these listed in order on the signoff card, or just call and ask the building official.

If you did not pull a permit, I'm sure you would prefer to get it in ASAP before the temps drop.