Has to be in the first 10 days to keep the virus from replicating before the cytokine storm kicks in.
Giving of monoclonals is not cut off by time, although 10 days is common. The real determining factor is blood oxygen level: Drop too low and the monoclonals are contraindicated because they'll likely then do more harm than good. With Delta,
some people can drop to an O2 of 90 or below a few days after symptoms appear.
Also, if a complication is already developing / in progress, monoclonals and most prophylactic treatments are not likely going to treat it.