The standard timeout is 250ms. This is appropriate for most commands, but rebooting takes longer.
I've found that, in practice, a 1000ms timeout is more than sufficient for the reboot command. This is easy to handle in the CLI - crystalfontz --timeout 1.0 power reboot-lcd - but it's currently a footgun.
On the other hand, actually fixing this means complicating the domain model by adding a second reboot_timeout, or even long_timeout. Or, I guess, I could set the default timeout to longer, outside of the advertised SLI threshold for standard commands. Hmm.