What percentage of profit to exit if a stock is making profit ?

This would be 'market timing', which is not possible, because we are no prophets.

Also the new cash money from the sale must be invested again, incurring trading fees, but particularly selection of a better investment then is difficult.

on - what I believe - is more boring and more successful, You could read here:

https://seedly.sg/questions/what-is-your-genera...

Depends on the outlook of the company? If its good can just keep holding on to it cus it will keep rising. S&P500 rises more than triple 300% for the last 10 years before it went downhill.

If you are talking more on trading, maybe around 5 to 10%? For every year you hold on to the stock, just add 5% (since CPF SA is 4%, you should aim to outdo it)