(cake stuff, I know, I know 
)
I would be on them the next day unless there was an agreed upon return date.
Is that unreasonable?
I think it would depend on the event...
Understandably, a wedding has a lot of clean-up and other stuff to deal with. A birthday, christening, etc... not so much.
Maybe you could send a polite reminder email or phone call just to remind them to get the materials back to you as soon as they can?
If these were pans I would expect that person to have returned them the next day after they used them (if they're local). I suggest you give them a call and politely ask if they're finished with them yet, as you have an order requiring them.....even then, you need them back!
I would request the items back withen a day or two after I knew the event they needed them for was done. I have borrowed some cake pans from family and have only kept one of them b/c she said to keep it until she needed it for something... but she doesn't bake much anymore and I have done her daughters cakes the last couple of years so.....
A phone call never hurts to find out when they will be returned...
That is so funny you knew what members were going to say to you about loaning baking equipment. 
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