For Popular sites, it takes couples of seconds to minutes to get indexed and listed. For most thing, even new posts on Taranfx get indexed in under 5 minutes. But sometimes it’s tough for Google to index and reach-out relatively new sites and pages.
To start with, Google started indexing and displaying realtime content from Networks like Twitter, Facebook but that doesn’t help much. Google Still doesn’t index the realtime content. Google News quickly learns popular stories among top sites and displays them in 2-3 hours. But the indexing and ranking them in Search is no where near.
Basically, publisher informs Hub about updates and these hubs will notify Google, everytime new content is published. Based on this even subscribers can be choosy on the topics they wish to subscribe to on the Real-time.
But all this would require big changes for Sites which don’t, already, support syndication options at this time, which I believe would refrain Google from implementing this to full landscape.
So, PuSH won’t replace crawling, but a PusH will trigger crawling.
But the biggest challenge is all about handling the Spam. Weired marketers would await for any opportunity to exploit. Definitely Google will have anti-spam solutions before it even hits Beta, after all Google learned alot from Google Buzz.