I've been using oxygen and aeration stone for some time now.
my rules of thumb have been:
-any OG under 1.046, I don't bother with oxygen.
-OG 1.050 to 1.060, hit with oxygen for 30 seconds
-OG 1.060 to 1.070, hit with oxygen for 40 seconds
-OG 1.070 to 1.080, hit with oxygen for 50 seconds, etc.
-boil aeration stone before use.
-use a gas hose that fairly easy to remove from the stone.....hoses don't take to boiling very much.
The amount of oxygen to OG ratio I mentioned above isn't necessarily optimal or ideal, but it's what I've set as a constant.
I will say, there is a noticeable fermentation lag period in high gravity wort when I don't use oxygen.