Ignoring the question of how long it will take to actually use it all up, I am prepared to concede the point that if the modern increase rate in CO2 can be extrapolated to burning all known reserves then that would put us at about 700 ppm or less (2.5 times preindustrial levels).  Now, there is a big if in there since we don't really know how existing carbon sinks like soils, forests, and the oceans will respond to increasing CO2.  Right now, these systems buffer the atmosphere so that the annual increase at Mauna Lao is actually significantly lower (by about 50%) than if you just calculate the increase expected from all the CO2 humans dump into the atmosphere.  Basically because CO2 has increased, these systems draw down more CO2 than they did in preindustrial times and hence partially offset our pollution.  Some scientists think this buffer will become ever more effective as CO2 rises, others think it will saturate and a substantially lower fraction of our waste will be absorbed.  Obviously, such uncertainties are important for figuring out max CO2. :::Taking for the moment a value of 700 ppm as a reasonable end point, that still seems potentially bad.  How bad probably depends on how sensitive things like thermohaline circulation are to multiple degree changes in temperature, and how big a feedback is associated with retreating sea ice and permafrost.  Not exactly a risk I am eager to take given existing climate uncertainties.  However, at face value it would seem to take some disasterous scenarios off the table.  For example, it is estimated that the wholesale deglaciation of Antarctica won't occur till 3-3.5 times preindustrial CO2 levels.  (This was presented at a conference of the American Geophysical Union, but I am having trouble finding a reference). :::I think your consideration of how high can CO2 levels really get is a valid one, and potentially very intersting.  However, given the uncertainties involved in such estimates, I would really like to see if we can find something published on the issue.  An academic paper would (hopefully) have spent more time thinking through these issues and work out what kind of a range of values is possible. :::   