Let's say we start with John's POV and then switch to Mary's, spending 2 days with her. Then, we shift to Cat's and spend 3 days with her. You need to keep in mind that when we return to John, 5 days have passed for him. It gets worse when you spend months with one character. How to keep track of this? First of all, don't worry too much about it in the first draft. You'll only make your head hurt. Since my characters spend a great deal of time traveling, I plan to make a schedule for them during my editing phase. It'll have where they're going, why, travel time and how long they'll be at their destination. While editing, make a "comment" or note about how long you spent with this character.This is a bit of a nightmare. I'm editing Chains of the Sciell Book 2 of The Merging Worlds Trilogy. The cast is much bigger than book 1. The timeline is longer. Book 1 only took place within a month. Book 2 spans about 3 years. To make it even worse, the characters travel a lot.
When you have a novel that spans a great deal of time, don't forget weather and seasons. I never thought of this until I started editing. It's one of those little things that can pull people out of the story. You're with a character for months and it never rains or gets hot or really cold. That's unlikely unless you've created a world where the weather is always perfect. You'll need to say that early on.
To keep track of time passing, I created a table in my favorite software Evernote. I'll keep adding to it as I continue editing my story. Creating a chart, helps you see how time flows between events and POV shifts. It also tells you if you need to adjust your timeline. As you know, in stories, events don't often happen one after the other. Two different events could be happening at the same time with two different characters.