You can create a calculated field in Data Management on the Company Object. Choose a number field and then you use a Date Diff formula with today as the start date.

You cna do the same thing in report builder too if you are not needing the field everywhere. 

Since you’re going to be doing this in the overview section of the success plan, I recommend adding the calculated field to the object that houses your renewal date, like john_apple suggested. 

I just added calculated fields like this last week to show a “count down” and it worked perfectly! Then you don’t have to worry about re-creating the field again since it will exist directly in the object

It honestly depends on your requirements.

If you are needing it just to display on one or a few reports, then do it in the report builder not on the object.

You do it on the object if you are needing it multiple places; reports, rules, DD’s, etc.

And yes, it does hurt to add another field onto objects if not needed. Maybe not a low amount when an environment is new. But when the environment has been around for years more and more fields are just added willy nilly and it degrades performance. And, there are some objects that have limitations on the number of fields you can add.

At which point you may want a ‘company attributes’ object with a Company lookup to store more custom “doesn’t have to be ON the company object directly but needs to be widely utilized and accessible from Company” fields.