Nvidia's denoiser is shit at small details like skin imperfections, because it can't distinguish between what's part of a texture and what's noise. There are ways to have a texture aware denoiser (cough, cough, Albedo), but Nvidia doesn't give a shit.
So far, it means denoised images tends to have a slightly plastic look, unless you literally runs the rendering for 99% convergence (upon which why are you even denoising to begin with). The result can be more or less acceptable, depending on what you strive for.
In any case the denoiser does not affect the convergence ratio, and can be plugged equally at any time, with no incidence on the final render. It tends to take a big amount of GPU ram when it is activated and to slightly stall your rendering at each denoising pass, but it isn't that noticeable. So you can just activate it at rendering pass 8 and keep it on.
TL;DR unless memory is a problem, if you want your denoising, just activate it and stop the rendering when it looks "good enough". But don't expect miracles in quality.