I'm afraid the only answer here is "as long as needed".
The story must captivate you, the writer, enough that the characters fight for their time share, appear for their specific purposes, linger in the background creating the mood, surprise us with their reappearances, break the mold, capture our hearts, capture your heart. You spend on each just enough time to have them fleshed out sufficiently, fulfill their role and then some - to let them be more than a literary tool, be a live soul too. This is not something that can be measured in pages.
If you try to apply a technical metrics like time or pages, engineer the timeshare given to each character, don't live the story with them but just construct a literary product matching given technical standards, you won't be writing a very captivating story.