The red line has been slowly falling from 2008 onwards, most recently the drops have been from an equal combination of rising rents and falling prices. The historical bound for price-rent ratio before 2000 has been between about 70 and 90. As an estimate, say the price-rent ratio is to revert to this historical bound in five years. What average annualized price change is required to do this?
Rents have averaged about 2.5% annual gain since the 1990s. Under this scenario, five years of -2.5% nominal price changes will put the price-rent ratio within historical bounds. To get to the middle of this range would require a more hearty -5% annualized nominal price change. If rental growth slows due to slow income growth or oversupply, price drops would need to intensify.