From the time you give your supplier your box/pallet labels to the time your product arrives and is checked in at an Amazon FBA warehouse can take anywhere from 1 day to several months (usually no more than 90 days).
SPD shipments will usually move much faster than LTL pallets. At a minimum, pallets will usually take a few weeks.
Factors that impact shipment speed include;
1. Supplier Speed - how quickly your supplier prepares and sends the package
2. LTL vs. SPD - SPD is usually much faster because you are relying on UPS vs. pre-scheudled tractor trailers
3. Current Capacity/Demand on Amazon's LTL Carriers - The more demand for Amazon's LTL carriers, the slower those carriers will be to pick up your good and the more stops they will need to make (to pick up other LTL shipments) on the way from your supplier to FBA. This is a major factor during times of high demand like the Holidays or the COVID Crisis.
4. Current Capacity/Demand at Amazon's Warehouses - The more product arriving at Amazon warehouses for checking, the slower Amazon's check-in teams will be to check in your product. This is a major factor during times of high demand like the Holidays or the COVID Crisis.
5. Distance From Your Supplier To The Warehouse