If God had wanted us to have nearly unlimited clean energy, He would have placed a fusion reactor into the sky.
(europe.pub)
15 years I think
This number gets lower every time I see it.
First, manufacturers typically guarantee their panels for 25 to 30 years.
Second, while we can extrapolate from existing data and perform accelerated aging tests, we're actually not completely sure how long PV panels last in the real world because the oldest ones from 1987 are still going.