If the stock is greater than 10 this updates on a 15 minute schedule, if below 10 we queue the update out to eBay instantly (but time could vary on this depend on when eBay process it - but would expect to see the stock updating in a couple of minutes)
Just make sure that the stock updates are turned on correctly. This can be seen by heading into configuration (Right top corner) selling > eBay and pressing leaving the don't update eBay option unticked
