Unfortunately, I'm pretty sure an unpaid item case can't be opened once 31 days since the date of sale have passed, and the sale essentially becomes void by eBay's standards 35 days after the sale (the fees are still charged if the transaction wasn't cancelled either by mutual agreement or UPI case), and the buyer will no longer be able to go through eBay's checkout to pay (they can still use bank deposit or pay a direct PayPal invoice, though, if you were still prepared to accept payment).  In my opinion, 4-6 weeks is too long, and I recommend you go to the resolution centre post haste to see if you can open a case.

In so far as how long should a seller wait... As long as they want to, really. For practical reasons, it's best to allow at least 4 days (you can't open an unpaid item case before then, and even then another 4 full days has to pass before it can be closed, so minimum 8 days before you can officially finalise a transaction without payment). I allow 5-7 days, depending on the item and state the payment terms in the listing, as that gives me a term of sale to refer to if a buyer wants to negotiate longer payment time frames ( I will allow extra time, but generally if the first extension is not met, I do open UPI cases and tell the buyer that I am unable to continue to withhold the item).