I think it will be 50 years or less considering the rate at which natural resources are being depleted.
Then many will lose faith in God (no matter their religion) because everyone expects God to solve their problems for them, rather than being proactive and taking responsibility for their own actions.
This kind of thinking is rampant in Watchtower Land (wait on Jehovah) and seems to be quite prevalent in the secular community as well (Our government will fix it). The thing is businesses react to supply and demand. If the majority of people on this planet rallied for renewable energy sources and stopped using non-renewable, things would change. As it sits right now though it easier for many to ignore the problem because it's an inconvenience to them and supposedly "out of their hands."
Although I do hope that it comes sooner in light of Revelation 11:18 "He will bring to ruin, those ruining the Earth". Which seems to be a promise of destruction of those by God who choose to purposely destroy those who willfully harm our beautiful planet for the sake of their own greed.
Peace and Love,