How long does standard flat rate shipping service take from china to Alabama?

There are a number of uncertainties and am not clear precisely what you mean by "flat rate". If you mean by a typical cargo ship then you would be talking about an average speed of 12 to 18 knots, whereas a passenger ship could go faster, most can do about 26 knots if they are on full speed ahead, but to save fuel costs they may proceed on reduced speed. But also a ship plying between a port in China and a port in USA can go by different routes, such as via Suez Canal, via Panama Canal via Cape of Good Hope (South Africa) or via Cape Horn which is the southernmost part of the Continent of South America. While you might assume a ship would normally go by the shortest route with cargo ships this is not always necessarily true. This is because in some cases a cargo ship may need to call somewhere else en-route to pick up or discharge cargo, whereas passenger ships may be routed via the most enjoyable seas and ports and pleasantest weather conditions, which may not necessarily mean going by the shortest route. But also you say China and what the shortest route is depends on where in China you are talking about as passage times and distances vary considerably according to which Chinese port you are departing from.

So assuming you mean a port on the East China Sea such as Shanghai doing 12 knots average speed on a "Great Circle" course across the Pacific and then through Panama Canal it would take about 30 days from departing China to arriving at Balboa, which is the western end of Panama Canal, then about 8 hours passage through the Canal, although sometimes ships may have to wait some time to go through. The Master of cargo ships usually plans a passage by the shortest route across a major Ocean like the North Pacific to get there in the shortest time, but the disadvantage of a Great Circle course is that it means going to northern latitudes where weather can be very bad and this does mean it can be very uncomfortable if the ship is rolling a lot in heavy swell, but seafarers are used to that and just accept it as part of their job. Passengers however want nice weather and calm conditions so a Master would probably do a rhumb line course to get the best weather but this puts on a lot of extra miles compared to a Great Circle course, but sea currents may also need to be considered.

So reverting to our cargo ship doing 12 knots she may also have to stop for such purpose as "bunkering" or refuelling. After Canal transit it would then be approximately 3 days steaming time from Cristobal on the east end of the Canal to arrival at the port in Alabama. depending which port in Alabama the ship is bound for and whether there would be any delay in going ion to accommodate tides. So in total with a 12 knot cargo ship we would be talking about between 32 to 33 days between leaving China and arriving in Alabama.

Considering now a passenger vessel that does about 30 knots, the passage across the Pacific might mean doing a rhumb line course. This is because weather that way is very much calmer and so pleasanter but means a significant increase in total distance. So we would be talking about approximately 8 days across the Pacific, the 8 hours Canal transit and then a day and a half to Alabama meaning a total time between China and Alabama of about 10 days. Ships can however be delayed by bad weather which can sometimes add a day or more to a passage time, which is why everything has to be stated as approximate.