It depends totally upon what you want to do. You can drive around the trail in one day. So you have to look at activities which interest you. The Cape Breton link above has sections on hiking, whale watching, music, artisans, museums, etc.
When are you coming? Accommodations from now till mid October are limited. The Celtic Colours Festival is from Oct. 6 - 14. The island will be hopping then. It is a great time to go but accommodations are limited.
Many people stay part of the time on the west coast (Mabou up to Cheticamp) and part of the time somewhere central (Baddeck or north and south of there.) Some keen hikers will spend time in the north based in Ingonish or north of there. But how many nights depends upon what you want to do in between.
How many nights and days will you be in Nova Scotia? Five? Then you will have three full days in Cape Breton and possibly two half days.
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