By definition, overclocking is "off-spec" operation. Being off-spec...there is no spec, or datasheet, for "maximal" overclocking rate.
EDIT adding: If you want to know what the potential is then your best bet is to ask the overclocking community who've pushed that part with overclocks. But always keep in mind that results vary, sometimes wildly, depending on setup (motherboard and it's VRM, cooling methods used, PSU, owner tolerance for stability and/or noisy fans, etc.) and silicon quality of individual CPU samples.