Short answer: up to 8KBytes/second.

Long answer:

Writing speeds will vary widely depending on the type of make/model card you're using. There are a lot of different factors that can affect the speed of the card.

To maximize the throughput to the card, try following these guidelines:

Open a file on the uMMC/uMP3 for writing, start the write command (for 512 bytes - e.g. on the uMMC "w 1 512"), then send the data as you get it. Of course, you should keep track of the number of bytes you have already sent, and be prepared for a delay after the buffer is completely full on the uMMC/uMP3.

Essentially, this is what you will do:

1. Open a data file for writing/appending (e.g. on a uMMC "o 1 a /data.txt").
2. Start a write command (e.g. on a uMMC "w 1 512").
3. Collect data, then send to uMMC/uMP3 (keep track of how many bytes you've sent).
4. After 512 bytes have been sent, it will take some time for the uMMC/uMP3 to commit the data to the SD/MMC card. You may want to buffer 5 to 10 packets of your data during this time.
5. Go back to step 2.

I hope this makes sense. In this way, you have the ability to use the buffer on the uMMC/uMP3 to store much more data at a time.

The time it takes to commit the data to the SD/MMC card varies widely with the speed/brand/size/format of card used. It is recommended to always check the module's response before sending a new command.