Kinda depends how your story is broken up and how long it is. Generally speaking, I think the rule of thumb is that small updates can be done at the end of a "section", and larger updates upon completion or in preparation for publishing somewhere new, or physically, or wev.
Small updates might be things like typos or grammatical issues that you notice or are pointed out, or if you need to rearrange a speech bubble, or if you forgot to add highlights on a panel, or if you pinpoint a wonky panel or two that you want to touch up.
Large updates would be like redrawings of multiple panels, or large format changes, or other heavily time consuming changes and amendments that would interrupt the telling of your story too much if you were to stop and do them midway.
For my story, I have it broken up into little self contained seasons where even though they share the same world and characters, each one is meant to be kind of standalone. I'm currently on break between Season 1 and 2 and so for my own comic, I'm thinking about going in to apply some of those large updates to Season 1 at this point. For example, I don't think I'll do too much re-drawing, but I do want to go through and change out the font to something easier to read, start preparing it for print, and create a vertical scroll version for webtoons. These will each take time and I don't know that I'll do them all all the way before starting Season 2, but I figure if I want to make these changes now is the time to do it while people aren't expecting updates lol