It depends on how you like the eggs.
I put mine in about 30 seconds before I take it off the stove into a bowl and let it sit in the bowl for a few min. The trick is to make sure some of the soup is covering the egg or it won’t cook at the top. This way it’s just runny in the yolk.
You can also eat it runny and add it once you take the soup/noodles off the stove and into the bowl. I do this sometimes and just break up the egg and it also tastes good.
You can put anything in it. I often put shrimp or scallops in with spam. I’ve also added shitakki mushroom or bok choi.
You can also add in a table spoon of soy sauce at the end, don’t cook with it. Soy sauce changes taste once you cook it.
American Cheese is the best. I also like the noodles “al dente” like @marasan
You can also add some dried seaweed.