Reasonable time to wait for payment?

[boogiewoogie]

An item sold yesterday but the winning bidder hasn't paid. I stated in my listing that I would despatch on 21st the latest after payment since I'm going away the day after.
I've sent the invoice about an hour ago to give the buyer a nudge but what if she still hasn't paid by the time I go on holiday? Even worse, what if she's completely forgotten about bidding to start with?

I think you are too quick off the mark - ebay will only let you open a non-payer dispute at the end of day 4 after the auction ended (which kind of implies that 4 days until payment is acceptable in ebay standards), so if it ended on Weds evening at 9pm you can open a dispute at 9:01pm on Sunday evening. Most of my buyers pay the day after and the remainder within 56 hours after an auction has ended so there's no reason to think your bidder won't pay - they just haven't paid yet.

Did you make it clear in your auction that unless they paid immediately the item could not be posted until xx/xx date due to holiday?

Wait until the latest possible moment before you go away and if still unpaid you could send the buyer a message to tell them that you are away until xx date and if they pay in the meantime it won't be sent until then. However if that wasn't made clear in the auction listing details be prepared for neg feedback.

The issue is that I won't be able to post it tomorrow as I have yet to receive payment and I stated in the listing that I was going away on the 22nd and that I would post on the 21st at the latest.

If I don't get anything tomorrow evening then yes of course I would explain that I would post when I am back.

That's sounds fine - if you explained in your listing then the buyer will just have to wait. You can open the unpaid item dispute on day 4 or later if they still don't pay and then you could wait until you are back home.