Can I gain a permanent right of way over my neighbour's land just by using it for many years without an agreement?

It's possible in principle, but far harder than people assume, and the details matter a lot. What's sometimes called a prescriptive easement can arise in Ontario through long, open, continuous, and uninterrupted use of a neighbour's land without permission, recognized through the common-law doctrine known as "lost modern grant" and connected to the Real Property Limitations Act, but this traditionally requires a lengthy period, generally around 20 years, not a handful of years of casual use.

The bigger catch is where the land is registered. If your neighbour's property is registered under the modern Land Titles system, which now covers most of Ontario, establishing a brand-new prescriptive right against a registered owner is largely unavailable. The whole point of Land Titles is a government-guaranteed register that long, informal use generally can't override. This possibility remains more realistic only on land still under the older Registry system.

Simply using a path or driveway across a neighbour's land for years, even many years, doesn't create an automatic right, and assuming it does is a common and costly mistake. If you're relying on longstanding use, get legal advice on the specific registration system and history before treating it as an established right.