making DNS changes (TTL theory)

[Jim]

Hey folks,

Being relatively new to dns, I'm a bit uncertain that when i make changes to 
bind, how to accurately im able to predict when the changes i make will 
replicate to dns servers across the world. I'm asking y'all for help because 
i've heard it said from people who manage the same zones, that it might take 
up to 2 weeks for changes to have fully taken affect across the net - which 
i am willing to contradict. I've always been under the understanding that 
changes made to RR's could take up to the value of my TTL for the zone. Let 
me explain how i draw my conclusion for dns changes now, and i'd be happy to 
have anyone correct me on this or offer any advice.

For zone1.com, my TTL = 2 days and my expire = 2 weeks. My expire value is 
only relevant to my slaves of zone1.com. If my primary name server is 
unreachable from my secondaries, the zone data will expire, and my 
secondaries are no longer able to provide authoriative answers for my 
zone1.com.
As for my TTL of 2 days, this value tells any non-authoritative name server 
how long they should cache my zone data for. So if a non-authoritative 
server has my zone data cached, then i make a change to my zone & reload 
bind, the countdown begins from the TTL and the client using this 
non-authoritative server can expect (worse case scenario) that in 2 days 
they will have an updated answer for all RRs in my zone. In which case i 
would be correct in assuming that in the worst case scenario, my zone data 
will have updated answers from all internet servers depending on the value 
of my TTL. Are both of these assumptions correct?

Any help would be appreciated :)
Jim