There are 10 year periods where it has doubled. But on average it's 15.
But I think over the next 15 it probably won't. Inflation is too low right now for that.
Guessing at these things is really pointless. Even though there are averages it's up and down decade by decade. Averages aren't what is going to affect us. Real numbers are.
The other thing I'm taking into account is that the higher it is, the harder it's going to be to double it. In other words, it's a whole lot easier to justify doubling it from $1500 to $3000 than it is to double it from $6000 to $12,000. A much bigger jump over 10 years or whatever years it is.