[BrandonLive]

Reboots are usually required for monthly "patch Tuesday" updates, though sometimes they are not. Out-of-band updates are rare but do happen in the event of a critical, urgent security update.

The OS only sets a deadline for the reboot (where it will force if you postpone too long) if the update includes a critical security update.

Firmware and core driver updates also require a reboot, but don't have deadlines and the schedule depends on the hardware vendor.

Oh, and if your machine is in the Windows Insider program, all build upgrades have deadlines so you hit the "forced reboot" much more often than a normal user.