since there is no bios, I doubt this will take very long to boot at all. In fact boot time will mostly be determined by how many services you are starting at bootup. Yocto linux. I imagine most applications of this device will start ssh, maybe a web server, or an otherwise custom developed service for the specific application. The bootloader may have a few seconds delay to allow you to interrupt it and change params.
How long doe it take to load a kernel and initrd from flash, unpack and execute?