No reboot needed. I have updated my hosts file regularly in linux, windows and macosx without needing a reboot.

Generally your browser will pick it up quickly as well. It doesn't cache host file entries in the same way as dns lookups so the effect of adding or removing is pretty much instant. I use Chrome mostly, so I am not as sure about other browsers.

No need to reboot, as soon as you change the hosts file the OS should immediately start to use the new version, the hosts file is the first thing the OS checks when resolving names, this is what enables this approach to work.

It takes immediate effect. I often have a terminal open and toggle commenting out an entry when testing something. As soon as you save it, it's good to go.

$sudo nscd -i /etc/hosts

also

$man nscd