Should I setup serveral instances of Arch to get the hang of it? How many? Well it depends on how good you are, but i'd suggest atleast two.

How does ArchLinux compares in gernal productivity compared to Windows? Well depending on what DE/WM you pick you can make your workflow more fluid with keybinds or by setting up stuff that gets loaded automatically and set etc..etc..

What habit changes does using Arch involve? Reading the reddit / sources to check if any updates will ruin some program you have. (GTK and such..)

What information pools beside of arch-announce and lists.archlinux and reddit should I see? lwn.net for security things

How long did it take you to set up arch? Well the first time I actually did it about 3 times in a VM and probably one more time but I can't remember. I do remember though that it was because I wanted an encrypted installation that it took a little bit extra work.

Any must have tools? Hmm... noscript for your webbrowser is a good add on. Can't really think of anything except things like screenfetch, archey to display information that might be interesting for you and if you're taking screenshots to share.

Do I have to spend 3 Hours to get feature X? Unlikely, but there are probably some rare cases where you have to wait for compiling and such that might take alot of time depending on your pc.