The logic is basically this, so the GW happens. Day or so later, they fight the black saints and ikki. Immediately after, the silvers. They go back to Japan, fight a second group of silvers. Seiya finally goes to the hospital, stays there for what can’t be more then a few weeks at most. A few days later, the gold saint fights happen within a day. So all of the start took about a month or a month and half. Poseidon is stated to be one month after the sanctuary. That whole flooding thing lasted 2 weeks. Hades was imminently after that. So seiya has been a saint for maybe three months.
ND is so soon after hades that Shun doesn’t even know where the other Mcs are, nor has he even taken his cloth out once. Considering the breakneck speed the og manga progressed at, I really doubt Shun waited months and months to visit seiya, y know?