Run this in Terminal.app
defaults write com.flexibits.fantastical2.mac DefaultEventDuration 1200
1200 = 20 minutes * 60 seconds
defaults
is a powerful command. You can change the settings of most mac apps with it, which is useful in the case above (where fantastical 2 gives you a dropdown of options that dont work for you) or in automating your system setup (like when you get a new machine)
The way I found this default was I used:
defaults read > ~/Desktop/defaults.txt
to print all my defaults into a file. Then I opened my editor a searched for "fantastical"
Once you find the "domain" you can find the "key" of the setting you are looking for and then you can change it with:
defaults write <domain> <key> <value> writes key for domain
Just typing defaults
will give you all your options.