Wow, so it takes a little over 6 days to ramp up to full rewards for blocks found right now...I was wondering why my rewards for those last 2 blocks were horrendous. But I'll stick it out since the blocks found are going to be that much sweeter once my rewards are in full swing.
Well, technically, it's actually that it takes 500% Diff of constant mining for your block Reward to reach it's maximum you can get per block
The 500% currently is ~146hrs, as you said, a bit over 6 days.
Your shares define how much you will be Rewarded each block and continue to be Rewarded in each block found up to 500% after they were submitted.
The 500% reduces the variance Rewarded per block, but also means it takes 5x longer for each share to get it's full Reward.
It doesn't reduce the Reward, it spreads each share's total Reward out over the 500% after it.