So far I read the documentation at https://partner.steamgames.com/doc/webapi/ISteamUserStats#GetGlobalStatsForGame and I got this:
GET https://api.steampowered.com/ISteamUserStats/GetGlobalStatsForGame/v1/?appid=779340&count=1&name[0]=TOTAL_PLAY_TIME&startdate=1567301084&enddate=1567381084
However, it returns
{"response":{"result":8,"error":"Stat 'TOTAL_PLAY_TIME' is not an aggregated stat"}}
How can I get the total playtime for any specific game/app on steam?
It should be possible, since sites like https://steamspy.com are able to display stats like the average playtime.
I never figured out if it is possible by only relying on the steam api.
However for my use case it was sufficient to use the data https://steamspy.com provides. (Thanks @Fliere)
Specifically I used the average_forever values, which can be queried via https://steamspy.com/api.php?request=appdetails&appid=779340.
appid being the steam appid of the game you want to get the data on.