It varies…you have a few options.
You could, if you wanted to, claim the money back through a paypal dispute, and the seller would have to show evidence of having posted it. if he happens to not have that, they’ll likely decide in your favour. Which means you get all your money back, including postage, and probably make an enemy out of the seller. I really don’t recommend this unless you absolutely can;t afford to lose the money AND the item, in which case it probably wasn’t such a hot idea to be buying stuff anyway.
I have had purchases go missing three times as a buyer, but never as a seller.
Time 1) a cheap item, US to US, so I wrote it off. The seller swore she sent it to me but that was about the end of it.
Time 2) about $10 worth of craft supplies, Canada to UK. The seller apologised and offered to send a new batch for free. When the new batch arrived, I sent her an additional $10 as a thank you - so she paid the extra postage and I paid the extra item costs. As the postage was also about $10 it seemed fair, and she was very grateful.
Time 3) about $35 worth of craft supplies, US to UK. When I reported that it hadn’t arrived, a month later, she immediately sent a new batch (without even asking me anything), which I appreciated but also made me wonder if I had been overcharged the first time! But maybe her company was so big that she could afford it.
The item that you bought - is it something that you could use two of? if so, you could ask the seller to send a replacement and offer to split the costs…but personally I would wait a while longer. I once sent myself a package from the US to the UK and it took about 7 months. When it finally turned up, it had split open in transit, was taped up, and had probably spent a few months on a post office floor somewhere.