@Darva1 there is no strict rule, but common sense would suggest, that you adapt your action to the value if the item. There are rice cookers available for 15€ or $ and others costing a fortune.
As you asked them about sending it, they may now expect to have some time to come up with a decision or finalise their plans when they may return. I made that mistake at the beginning and had a collection of worthless items, mostly clothes, some prescription drugs, perfume etc, that the owner asked me to keep till “next year”. Which is obviously never. Even if they do come back and you tell them where you stored the stuff, they manage to forget the thing again 😉
I would write to them a last time, to let them know that you need to remove the items from the flat and in absence of a forwarding address and the postal fees sent via airbnb in advance till ... , you will donate it to ....
come up with a nice destination, charity collection or fundraiser sale and they will feel better about loosing it. It’s probably not worth the postal fee and definitely not worth your time to write them many times. (Include a fee for posting the stuff if you calculate postal fees).
i would put ot into the garage for a month longer though.