My cats had the string pulled apart in five minutes. But the suction cup and plastic arm are strong, and it's easy to tie other toys onto it. I wouldn't let cats play with it unsupervised since ingesting the broken string could harm them, but it's fine to leave up if you replace the elastic with a good sturdy ribbon.
Surprisingly strong for big cats
My cats love this. It’s perfect for our patio windows and was bigger than expected. The window sucker is very strong and my 5.5kg cat has not managed to pull it off yet, despite his best efforts.