I watched my dad do 30-60 days every few years growing up. He always said how clear it made him mentally and the benefits to his health also was visible (until today he mountain-bikes across South East Asia during winter and he is in his late 70ies).

Longest I did was 40 days. Started out with 10 days a few years earlier which broke my misconceptions about if I would be able to do it. Especially the 40 days has been a healing journey for me both physically and mentally. It also gave me massive confidence about my body and taught me more about my limits than what decades of running did. I wouldn't do this long again because I lack the reserves today. But 10-20 days every few years is no issue.