Forget about REISUB. I don't know who invented this, but it's overly complicated: half the steps are junk. If you're going to unmount and reboot, you only need two steps: U and B. At most three steps E, U, B.
Alt+SysRq+R resets the keyboard mode to cooked mode (where typing a character inserts that character). That's useful if a program died and left the console in raw mode. If you're going to reboot immediately, it's pointles.
Alt+SysRq+E and Alt+SysRq+I kills processes.
E sends processes the SIGTERM signal, which causes some programs to save their state (but few do this). If you do
E, there's no fixed delay: typically, after a few seconds, either the program has done what it was going to do or it won't do it.
I sends processes the SIGKILL signal, which leaves the system unusable (only
init is still running) and is pointles anyway if you're going to reboot immediately.
Alt+SysRq+S synchronizes the file contents that are not yet written to disk.
U does that first thing, so doing
U is pointless.
Alt+SysRq+U remounts filesystems read-only. If you can see the console, wait until the message
Emergency Remount complete. Otherwise, wait until disk activity seems to have died down.
Finally Alt+SysRq+B reboots the system without doing anything, not even flushing disk buffers (so you'd better have done it afterwards, preferably as part of Alt+SysRq+U which marks disks as cleanly unmounted).