Assuming nothing breaks the plutonium RTGs will decay below usable production circa 2025 or so. RTGs never exactly stop producing power but their source decays in accordance with the half life, usually just under a century for plutonium RTGs, and the thermocouples degrade too. RTGs aren't very efficient, they're just very simple/durable, and you eventually drop below a certain power threshold. The thing could break at any moment of course, and not all of the systems still work, frankly its damned impressive engineering that so much functionality remains.