Curious how much gold you'd sit on if they dated the ~150 card expansion to releasing in around ~3 months, and said they were selling expansion only packs for 100g a pack (and none of the expansion cards could be obtained from crafting regular/core cards in to dust). Figure someone willing to save 3000+ gold for a PVE adventure probably will save a decent number for an expansion too.
Once they give a date for the expansion, I'll probably stop spending gold entirely myself, until it hits. I'm also around 3000 gold myself, and probably will continue to save until they say what the PVE will cost.
Well, I also have most of the normal cards already. That helps a lot. I don't have them all (no Hungry Crab or any Ice Blocks, amusingly), but still have most of the cards and pretty much every useful Legendary already. 2990 gold and 3220 dust stocked. I'm okay with going a bit longer without Lorewalker Cho, Millhouse Manastorm, or Nozdormu, fun though they could be.
It also helps that I've grown to despise opening one pack at a time and enjoy a bare minimum of 10 at a time.
That being said, if I had maybe half the cards I have now? No way I'd have been able to stockpile. Far too impatient.
That's going to hurt Blizzard the most, the longer they wait, the easier it is to stockpile cards and as a result, gold.
[edit] To directly answer your question? 4k gold.