Having just gone through this, NVC is S-L-O-W right now.. it was 100 days from NOA2 to NVC case number (several inquiries and a letter to the senator).
Yes, the I-129F will expire 120 days after NOA2, but the embassy has the discretion to extend the date, but they can't until they have the document (CEAC status of at least 'in transit' or 'ready')
Also, using logic with USCIS and NVC about 100+ days to embassy and 120 day expiration does not work.
My recommendation is have the USC monitor the progress, call USCIS to see if you can find out the ship date to NVC (at about NOA2 +80 days) - it may take several calls; then line up your senator if you go over 90 days of no answer from NVC. This is a HUGE babysitting exercise and literally the most stressful part of this whole process.
Also, depending on your embassy, you can get a lot done during this wait - DS160, MRV, I-134, etc.