You probably would need to factor slightly more time as you will be dealing with City of Stirling for a DA.
As a overall urban development strategy for Perth metro I don't think the zoning will be revoked to R20. For a detailed insight read this...
Just a observation last year similar size blocks were selling for $380 in balga that's a $40000 price rise per unit only on the land cost on a triplex site. Thats good return holding land for a year.