I structure my comic in 'arcs', I guess
They can have any number of pages or climactic scenes; I guess they're more defined by 'when I've said all I wanted to say about this character's (current) plight', that's when an arc ends and then I introduce another character to focus on, which marks the start of a new arc.
My entire comic doesn't really have a 'beginning, middle and end' because it's like 4 smaller plots strung together that have their own 'beginning, middle and end', so an 'arc' would be one of those 4 'component plots' that you can map a 'beginning, middle and end' onto.
I also find having a structure keeps me motivated, because the end of an arc is far closer than the end of the whole comic and I can just taste it :'D Since each arc has direction, it also keeps my story from 'kind of running together after a while'. But I guess I didn't define this structure in order to help with my work habits; it's more like I discovered my comic naturally falls into this structure when I was trying to figure out what I want to say with my comic and where it's headed 