Every jurisdiction publishes a review target. Very few of them are a useful planning number, because the target measures one pass through one department — and most sign permits are not that.
1. How many departments see it
A single building department reviewing a wall sign is a two week job. Planning, then zoning, then building, then public works in series is a two month job with the same drawing.
2. Whether anything triggers a hearing
Historic districts, design review boards, planned unit developments and variance applications all convert an administrative approval into a scheduled meeting. The review time stops mattering; the meeting calendar takes over.
3. How complete the first submittal is
This is the one you control. A package that comes back for corrections restarts the clock, and two correction cycles will beat any expedited review fee you could have paid. It is the single biggest lever available, and it is entirely upstream of the jurisdiction.