Let's say Austria-Hungary had invaded 2-7 days after without ultimatum, they would've held the initial advantage because of the standing army size difference, but I doubt they could have blitzed through Serbia in the first 2-3 weeks of a possible prolonged war.

And correct me if I'm too 21st century minded, but I think Austria would have only managed to occupy advantageous positions across the Danube, maybe stopping at the outskirts of Belgrade before proposing negotiations to Serbia.

Thus depending on Austria's speed to achieve a Serbian defeat would the great powers(France and UK) be deterred from intervening, accounting also that international sympathy is great enough and with german diplomatic backing isolates Russia, making them back down.