Over time solar panels degrade, how much reduction in efficiency is expected? And at what point does this need attention?

Over time solar panels degrade, how much reduction in efficiency is expected? And at what point does this need attention?

Most reputable suppliers will have warranties which promise the panels will still have at least 80% of their nominal capacity after 25 years.  But if you pay extra for the very best performance warranty, these can state they will have 92% or more of their nominal capacity after 25 years.
However, most panels should exceed their warranty.
If they are less efficient than this within warranty, then you should contact your supplier.