This is completely dependant on the Class of SD card you are using.
A Class 4 card, which is the minimum recommended has an average read/write speed of 4 MB/sec.
If you spend a little extra and buy a Class 10 card, you should find that the boot time is approximately 25% of the Class 4, as it should read at 10MB/sec.
Using finnw's estimate that 24 seconds pass while using a Class 6 we can guess this could be reduced to 14.4 seconds with a Class 10 device.
Classes 2-10 will give you boot times something like this:
With the release of new hard float distributions boot time has been significantly reduced. In addition, Arch Arm Linux is now using
systemd instead of
init which starts processes in parallel and is considered to be much faster. As a result my Class 10 SD card now boots Arch to a login prompt in about 6-7 seconds.