How do I know when to sell a stock? Should I go based on the percentage profit made? Same for losses, how much percentage loss is a gauge for me to sell?

This question has no answer.

Probably (using the right benchmark) stock picking is not possible over long-term successfully

Depends. I usually do trading. My cut loss is always 20%. But if I do value investing, I would only sell if the business fundamental changes.​​​

A million dollar question! I think more money has been 'lost' in gains than in losses cause of this!

Head back to the fundamentals and compare it against the watchlist you have on hand.

I recently cut a 1/3 position with a 15% loss without hesitation as i felt that the companies on my hand are of much higher quality. No right no wrong.

I recommend you read this book The Art of execution by Lee Freeman-Shor on the execution part of stocks.

I doubled down on a huge position of mine after it dived 50% cause of this book and now its up 35%. ​​​

You shouldn't sell based on price actiosn! Take notice of the valuations and fundamentals instead.