It's pretty simply really. GP will continue dropping until Jagex finds a way to address the gold farming problem and the already trillions of GP in the game. I recall saying gold would be 0.25 when it was 0.50 and I think it would still drop to .10. As for a time, probably a year and some. Look at the trends (old GP selling threads) and try to establish a curve.
Meou that's not true for two reasons:
1. Profit margins are larger when GP is cheaper. For example, GP sellers used to buy at .7 and sell and .85 but now they can buy for .2 and sell for .3
2. People usually spend their whole budget ($20,$50, etc) - not just a mil amount- so they'll receive more GP but resellers will be making more profit.