I have no idea of any radar/ADS-B down time...
My last shift... my most significant RADAR failed and it was off for about five hours before it was fixed.
When I was working in Australia, the main RADAR that I relied upon would be shut down every year for a "grease and oil change" as we used to jokingly call it... yes, a RADAR has moving parts, in fact, the Mother-of-All moving parts... the RADAR head itself.
Like all moving parts, it needs to be lubricated and bits of it have to be replaced routinely.
Like all moving parts... it has a higher probability of failure... and it does fail.
Like the night shift when the Automatic Emergency Stop was activated... for no apparent reason. It just stopped turning... and I lost my RADAR picture. A tech was called in the middle of the night and he drove up the hill to the RADAR site to ascertain why it had stopped, but couldn't find a reason.
ADS-B = no moving parts.