How long can a denier of AGW hold their head under the sand without suffocating?

I don't think there's any end to it. The great part about global warming denial is that the deniers will be dead before we face the worst consequences of our GHG emissions. It's part of the tragic hubris of deniers (see link below) that they'll be able to get away with their denial, and it's only future generations who will really face the consequences.