Crit + gear effect came out to roughly 30 "+ crit" required to see a +1% increase in your actual rate when tested on R45 mobs in a 9000+ trial test Seiken and I performed. This was tested on melee swings, but we then performed a less rigorous test that seemed to show magic and physical crits follow similar patterns and formulas.

Given this, if you maxed out say a ~120 crit build, this would only give you a 4% true increase in your natural crit rate. This is assuming the per-point increase doesn't scale down as you increase dLVL.