Read the following passage carefully and answer the question that follows.

Rooftop solar power has quietly changed the arithmetic of electricity for Indian housing societies. A decade ago, a society that wanted solar panels faced high equipment prices, few trained installers, and banks that treated the technology as an experiment. Since then, panel prices have fallen sharply, subsidy schemes have simplified paperwork, and net metering has allowed societies to set off the power they generate against the power they draw from the grid. The economics are now straightforward: a typical urban society that installs rooftop capacity recovers its investment in four to six years through lower electricity bills, after which the panels produce power that is nearly free for the remainder of their twenty five year life. Yet adoption remains uneven. Societies with large, unshaded terraces and decisive managing committees move quickly, while societies with water tanks, mobile towers and disputes over terrace rights may never begin. Installers report that their hardest conversations are not about technology or cost but about who controls the roof. The lesson many state agencies have drawn is that the next wave of rooftop solar will be unlocked less by cheaper panels than by clearer rules for shared spaces, model resolutions for housing societies, and single window approvals that shorten the journey from quotation to commissioning.

According to the passage, in how much time does a typical urban housing society recover its investment in rooftop solar?