Couldn't say for sure. Probably a couple of years at most.
It surprises me that people thought the glass quality was poor. I think it's great. I even looked through a Zeiss Duralyt today and thought, "yeah glass is about the same". To be honest, for something that's costing around £700, you'd expect the glass quality to be at least entry-level-top-end wouldn't you?
Who knows? I could be waxing lyrical about an optic that you or anyone else might think is rubbish for the price (I doubt it mind ) but for me, the best I've ever owned in terms of glass quality was a Bushnell and this is far better. I'd love someone else on here to have owned one and add their own perspective.