Search handles probably an order of magnitude more changes per second than GMail+Docs combined
But if the index doesn't update you won't notice.
Yeah, search isn’t an OLTP system.
Offtopic, but it blows my mind seeing <1 min old StackOverflow comments indexed by Google search, the sheer amount of work going on behind the scenes to make that happen.
I'm pretty sure this only happens when the site is actively pushing updates to google via their webmaster tools. I think your sites importance would also impact how fast google indexes it.