First, goodkarma, I would say start with 100mg if you've never done it before, then go up to 200, then 300, then 400. I personally enjoy 300-350mg the most (when I have no tolerance) but some people I know get really high off of 100, some take 4-600. But if you're really sensitive, you'll regret going to that high a dose -- terrible itchiness, nausea for sure, maybe swelling and/or abdominal pain.
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What? That's doesn't seem right. But I think I remember seeing this on some random page though a long time ago... Shit, it's one Erowid and Drugtext even now. Hmmm, doesn't seem right to me -- I and many people I know routinely ingest anywhere from 400-600mg codeine at once, and that's with no tolerance -- and even at these doses, there's not much in the way of ill effects, just itchniness. I can't imagine the LD50 being that low!
From the <URL=http://www.inchem.org/documents/pims/pharm/codeine.htm>Poisons Information Monograph<URL>, the rat LD50s are 54mg/kg IV and 266mg/kg subq. For a light (50kg, 120lb) human, that's almost
3 grams IV or
13 grams eaten. Even with interspecies scaling, that's 1.1g IV and 5.6g eaten. And for a larger person the doses would be bigger.
Looking at clinical data (this is roundabout alas, couldn't find nice papers) it looks like the toxic levels of codeine found in overdosed humans ranged from 2-8 mg/L[1] and 22 mg/L[2]. The volume of distribution of codeine in man is 5.1 L/kg[3]. Blood levels must also be adjusted by oral bioavaibility, ~40%. There's also the possibility of posttoxicity metabolism, but I'm not even gonna worry about that (it pushes the numbers higher.)

That gives for a 50kg adult 1.2 to 5g, 14g. For an 80kg adult, 2 to 8g, 20g. These are much higher than the Erowid values.
It would be great if someone could find any good authoritative/respectable medical references on this. But at this point, based both on personal experience and journal research, I'd say the
minimum overdose range is ~1 gram for MOST people and probably the LD50 is closer to
5 grams.
[1] Gerostamoulos J, Burke MP, Drummer OH. (1996) Involvement of codeine in drug-related deaths. Am J Forensic Med Pathol 17:327-335.
[2] Kintz P, Tracqui A, Mangin P. (1991) Codeine concentrations in human samples in a case of fatal ingestion. Int J Legal Med 104:177-178.
[3] Findlay JW, Butz RF, Welch RM. (1977) Codeine kinetics as determined by radioimmunoassay. Clin Pharmacol Ther 22:439-446.