How to increase time of holding buttons to take screen shot on Samsung Note 8
To be honest, I don't if it is possible to increase the length of time it takes to capture screenshots using buttons.

However, since you get unwanted screenshots when using the buttons, I advise you try using other methods that your device is capable of.

A method that may help you is to use your hand to swipe and take a screenshot.


Goto Settings -> Advanced features -> Enable "Palm swipe to capture".
Open the app you want to capture.
Swipe the whole screen with the side of your hand.