How can I change banner notification time on a MacBook with OS X 10.10.5?

You cannot easily customize the time anymore on current os since the preference has been removed since MacOS 10.10 Yosemite.

If you want your notifications to stay on the screen until you dismiss them, you can do the following:

Go to System Preferences > Notifications

Select the notifications you want not to be dismissed automatically. Change the "alert style" to "Alerts" instead of "Banners".

I don't believe @oa-'s answer is correct.

I am running Catalina (10.15.2) and was able to get a slightly modified method to what is mentioned in the link from OP working just fine.

First, you need to specify -int before the number of seconds parameter:

defaults write com.apple.notificationcenterui bannerTime -int 2

Otherwise, it's written as a string which won't be recognized as valid argument for the number seconds.

Before it'll take effect, you'll also need to restart the NotificationCenter process by signing out/in or running:

killall NotificationCenter