Odd question - but mine takes about 2 minutes 45 seconds (going by when the web interface says it's done), which is quite a bit longer than I'm accustomed to.
I'm wondering if it's something particular to my setup, or just normal for this router.
I'd say 3-4 minutes on average with pretty much everything on except Vulnerability Protection (which causes havoc for me, wan disconnections, both WANs showing down while still having internet access on secondary WAN, etc). About twice as slow as the AC66U.
OK, apparently because I had the T-Mobile version of the router, boot times were longer. Now that I have the regular 1.0.2.0 (US) bootloader, it's quite a bit faster. The web UI still shows rebooting for quite awhile, but wifi is back & fully functional within 75 seconds. Spiffy.