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".