Well, the half-life of caffeine in the body ranges from 2.5 to 4.5 hours. It's worth noting that women on oral contraceptives have impaired caffeine elimination [
reference]. Withdrawal symptoms should begin 12 to 24 hours after last consumption, and peak at 20 to 48 hours. Withdrawal lasts for about one week [
ref]
after 15 days without caffeine, previously caffeine-dependent rats had returned to normal numbers of adenosine receptors in the forebrain, but still had significant up-regulation in the cerebellum [
ref]. In humans, one adenosine receptor subtype, A
2A (one of the 2 for which caffeine is an antagonist), returns to normal numbers after 48 hours of withdrawal [
ref]. A
2A receptor stimulation does induce sleep, btw. Considering it's short withdrawal time, A
2A is probably the receptor type that we would be interested in here. And you
would have a different experience than someone who has not been tolerant because, theoretically, you will still have caffeine tolerant levels of A
1 receptors.
After intake, caffeine reaches peak levels in the blood after 15 minutes to 2 hours. So yes, it would start to have an effect within minutes.
And since I'm on a spider kick, poor spiders:

And video:
http://www.youtube.com/watch?v=sHzdsFiBbFcI should still research the two receptor types and sleep cycles.