The regulation is specific about the mechanics, and they are not intuitive.
For national interest waiver petitions, the timeframe commences on the date USCIS receives all prerequisites for adjudication together with the prescribed form and the fees. 8 C.F.R. section 106.4(f)(2)(ii). Not the date you send it, and not the date the fee clears.
Within that window USCIS must take an action, but the action does not have to be a decision. The regulation requires USCIS to issue an approval notice, a denial notice, a notice of intent to deny, or a request for evidence within the premium processing timeframe. 8 C.F.R. section 106.4(f)(1). A Request for Evidence satisfies the obligation.
And if that is what arrives, the clock resets. If USCIS issues a notice of intent to deny or a request for evidence, the premium processing timeframe stops and recommences with a new timeframe on the date USCIS receives the response. 8 C.F.R. section 106.4(f)(3). A premium case that draws an RFE is a case with two 45-business-day windows around however long you take to respond — the single most common way a premium filing turns out slower than the buyer expected.
If USCIS misses the window entirely, the fee comes back. With narrow exceptions, USCIS refunds the premium processing service fee but continues to process the case if it does not take one of those actions within the applicable timeframe. 8 C.F.R. section 106.4(f)(4).