My husband has done all of the real BitCoin research, and while I have kept up fairly well, there is a question that I'm trying to figure out that he has no answer for nor willingness to try to come up with an answer.  And it's quite possible that either he or I are misunderstanding something basic making my question irrelevant or complete nonsense.

He says that if I get a (for example, not attempting reasonable numbers) 1Gh/Sec device that earns $10 in a given time period, because of the change in equation difficulty, it will eventually earn only $5 over the same period.  I have no name for this phenomenon, but for the sake of this message, I'll call it splitting.

What I'm trying to figure out is a VERY rough approximation of how often this splitting occurs.  I understand that it depends on lots of different factors.  I understand that changes in the number of miners, the equipment, etc. will change the time between splits.  I understand that it's likely that it's not an even frequency, likely it's getting more frequent as time progresses.  But is it multiple times every day or every five years or once a year or once a month?

Obviously, if I'm planning to count on the income earned from the miners to any degree, I'll need to double my equipment about that frequently.  I'm trying to figure out how much (percentage) I'd need to reinvest in equipment to keep my earnings approximately the same.

I tried to look at some graphs, but since I don't even know what I'm looking for, I couldn't begin to calculate it.  Any assistance in this regard would be greatly appreciated.