For me, I can usually manage about five or six teas, coffees or diet Coke and after that, I seem to have a restless night. Of course, with nagging thoughts and Freddy, the two may not be related.
For me, I don’t drink caffine after 12 noon.
It’s an arbitrary time, but I noticed a few years out of college, that continuing to drink Mountain Dew in the afternoon ment coorelated strongly with my not getting good sleep. I started limiting myself by time of day, and have had fewer problems with it since.
For quantity… I haven’t ever ingested so much caffine that I wasn’t able to flush it out before bedtime, as long as I followed the above timeline.
-Doug
I can drink as many caffeine loaded drinks, or eat as much caffeine loaded food as I can possible stomach, up to a minute before bed, and still sleep soundly.
Caffeine makes me sleepy and a little giggly, kinda odd and what not (I call it my “fuzzy” state)and I am completly addicted. During the summer, and again around December, I was up to ten mugs of tea per day. Not healthy, I know, and Jester didn’t seem to like it, but c’est la vie.
So, yeah. Cafeeine doesn’t bother my sleep patterns at all. It actually helps them out… There is certainly something wrong with that…
I’m with Deiket, I can drink a Mt. Dew and fall right asleep.
'Course, I have sleep apnea so I never get enough sleep no matter how long I’m “asleep” and can literally fall asleep in a matter of minutes, usually before the second song on my stereo starts. YMMV
I sleep horribly, no matter what I drink, however, I did cut back my coke intake. I started having heartburn in the wee hours, waking me out of a sound sleep. I was used to drinking three or four cokes, I cut it back to only one, and having that one in the morning. And poof, no more heartburn!
But, as a doc, I bet you * already knew that! * 
I’ll usually have 6 cups of coffee when I wake up, plus maybe tea or soda or chocolate during the day. I can have caffiene-y things within one hour of going to bed, and still sleep fine.
I think it’s genetic. My grandma drinks coffee all day–several pots worth–and has no trouble sleeping. She sometimes takes a cup of coffee to bed with her to sip while she reads or watches TV or whatever, then goes to sleep like nothin’. I’m not quite to that stage of caffiene tolerence yet, but I’m working on it. 
An ungodly amount! Paradoxically, while coffee wakes me in the morning & keeps me going in the afternoon, I find it doesn’t affect my sleep at all. I don’t usually drink it after 3pm or so, unless I go out to dinner & have coffee after a meal. But even when I do drink it at night, I still fall right asleep.
Does anyone remember those 3 litre Coke bottles that were popular around 10 years ago? When we played our marathon Monopoly tournaments in university, each person would bring one, and we would invariably finish them. I never had sleeping problems, except for getting up to take a leak once during the night (or day). These days, I usually have a Pepsi slurpee (1.9 L) starting around 7pm, finishing around 11:30; I usually fall asleep within 15 minutes of bedtime.