The first time was by the late John Wimber when he said (on his church planting tapes, if I remember rightly) that, in his view, a local church should probably see itself lasting about 20 years before it either closes or undergoes such substantial and fundamental change that it, in effect, becomes a new church.
The second time I heard it was today when Steve Timmis, leader of the Porterbook Network and Western Europe director (or something equally impressive) of the Acts 29 Network, made the following statement on his twitter profile:
"Every church is 'designed' for a specific culture & generation. It has a 'sell-by' date which, if ignored, leads into institutionalism."
Meanwhile, I'll try and get some further clarification from Steve Timmis about why he believes this.