OK, solved it myself. I changed the autoextend limit temporarily to 92% as described in the thread mentioned above so I could start another AppVM. When I deleted some data there (that I had on another PC anyway), the size of its LV as displayed via sudo lvs in a dom0 terminal shrinked immediately. I then shutdown this AppVM and deleted the two backup snapshots of its LV using lvremove and this reduced the pool usage to 85%, so I’m fine now.
Then I noticed the “Data%” column for the LV of the AppVM which originally caused the issue also was lower compared to its backup snapshots, so it did shrink!
So the answer to my question above is: Actually immediately. You don’t even need to issue an fstrim. However, the freed space might still be in use by a backup snapshot – and that’s why the Qubes Disk Usage Monitor is not going down. 
PS: I just saw your reply, @apparatus Thank you, you identified the same issue! 
I understand these backup snapshots are there to save people’s life on a daily basis, so I’m glad they exist by default, even when they caused trouble to me. So, I hope this thread helps others to understand the issue and solve it, too. 