Minimum time to write all memory of DDR3 (micron)
If you do overlapped burst writes to all of the banks in sequence, it's possible to get about 80-90% of the raw data bus bandwidth as useable transfers. Most of the overhead for any given bank is "hidden" behind the data transfers to other banks. This is the approach we use when buffering high-definition video streams, which involves a mix of reads and writes from multiple requestors.

With a 16-bit DDR bus running at 800 MHz, you're transferring about 2.6-2.9 GB/s, so you should be able to write the whole thing in less than 0.4 second.