Some years ago I had to do several DNS changes over the course of several weeks as I moved bits of equipment from one data center to the other. At the time that I did this, about 95% of the nameservers in the world seemed to respect the TTL value, and about 5% ignored ours and made up their own. In other words, 95% of the traffic moved within the 15-minute TTL that we defined. Another 3% made it in the first hour, 1% in the first day, and a few stragglers took up to three days.
(Yes, OK, I'm confusing percentage of traffic with percentage of nameservers. Please insert handwaving.)
This was in about 2001, though, and we were using dinosaurs to transmit packets through the tubes. My guess is that today's nameservers are better-behaved, and there will be less of a problem with stragglers. Does anyone have a feel for what percentage of traffic will switch within the defined TTL these days? Are there still many nameservers out there that ignore TTL?