I would set it to a short period like 10 minutes and refresh it every 5 minutes as long as the window is detected open. So just in case something breaks, the window open mode will deactivate itself soon.
I guess the TRVs will still try to automatically activate the window open mode on temperature drop, so to me this features is actually more about deactivating the window open mode once I know the window is closed instead of waiting for a preset timeout that is doomed to always trigger too early or too late.
One issue here is the unceraint delay of communication between the Fritz Box and the TRV. I guess to save battery power, the TRV periodically (and maybe triggered by events like user interaction) initiates communication to the box and then sleeps for several minutes. Thus we will probably observe the same delay no matter whether we send “off” or “window open”.
But still, I’d like to see these features (window open and boost state read and write) exposed to Home Assistant. I put the “Fritzbox” in the title in braces because I think this also applies to other climate devices and should be considered a generic (optional) feature of the climate device representation.
PS: @sevorl and @fragger please consider also voting for this thread to raise attention and direct more users to vote for the github issue.