Is there are 'rule' one can use and does that rule vary for large cap stocks like BNS or a small cap stock like PNE which I also own?
Good timing of the market typically involves buying when assets have declined and selling when assets have appreciated. In the context of BNS, if one is trying to 'time the market', ideally buying at a lower price to sell higher would be the right approach. Of course, this practice is difficult to accomplish which is why the saying of 'time in the market is better than timing the market' exists.
There are many different rules investors can use; if a stock doubles, some investors like to sell half so that their investment is now purely a profit, or trimming at certain percentage gain intervals. Our favorite is trimming positions as they get closer to a 7% position, this helps to take some profits off of the table and rebalance the portfolio. It is certainly a personal decision and varies based on the stock - for example, a smaller name like PNE would experience more volatility than a large-cap, and this means being willing to let a position drawdown in price and potentially adding to it if warranted.
Largely, we prefer a 'buy-and-hold' approach, with a long-term timeframe in mind, letting winners run as much as possible while still being comfortable with a position size, and selling stocks based on fundamental changes to the company, rather than just price changes.