I follow a simple rule of thumb about racking to a secondary.
Less than 1.070 starting gravity, I usually don't rack to secondary, unless I'm adding fruit or something.
Greater than 1.070, I rack to a secondary. I don't have a particular timetable as to when to rack it though. I travel a lot for business, so my work schedule usually dictates it. In a perfect world though, I'd rack it at about 75% to 80% through the fermentation.
As an example, if I brewed a batch with a theoretical starting gravity of 1.100 and an expected finishing gravity of 1.020, my expected total drop in gravity through fermentation would be 1.100 - 1.020 = .080. I would multiply the expected drop of .080 x 75% = .060 drop to my rack to secondary. Thus, I would rack to secondary at 1.040 gravity. 1.100 - .060 = 1.040.
Examples, from what I have currently going:
I have 18 gallons of beer in fermentation right now.
13 gallons were racked to secondary.
8 gallons because that one had cacao nibs, vannilla beans and syrah grape pumice in the secondary. It was also aged in a bourbon barrell for 9 months. It was a club brew of two different beers (a RIS and a Belgian Strong), so all of us racked to secondary. Mine (8 gallons of the RIS) was racked about 80% of the way through the expected gravity drop. We all met at a friends house and racked them to the bourbon barrell, so they were racked three time (to a secondary, then to the bourbon barrell and then each of us got our share racked back to our kegs/carboys at the end of the barrell aging time).
Some of this will be bottled as is, which is a clean beer. Some of it will be saved to blend with the Oud Bruin or one of the two sours I'm creating for the Oud Bruin (described below).
5 gallons is an Oud Bruin on 9 lbs. of fruit. I racked the beer onto the fruit for the secondary and left it on the fruit for two months. It's now aging, before I create an unfruited soured version to blend it with.
I have 5 gallons in primary that had spices (ginger, allspice, cinnamon and clove) at the end of the boil. It's about 90% of the way through fermentation (bubbling about once every 2 minutes or so). It will be racked to secondary today, as it had a starting gravity of 1.087. I've recently raised the temperature on this one from 64F to 70F.
I don't have pure oxygen, as I use an aquarium pump with a diffusion stone. I give my wort 30 minutes of time with the aquarium pump for oxygenation. I also am religious about adding yeast nutrient with 10 minutes left in the boil, no matter what beer I'm brewing.