If I try to unlock my Samsung Galaxy A3 (Android Version 5.0.2) with an incorrect password 5 times I have to wait 30 seconds before I can try again.
I want to increase this waiting time: how can I do that?
As suggested by others in their previous comments, there is currently no application which would do that for you without requiring your phone to be rooted first.
LockScreen Tweakbox (which is a module for the Xposed framework) can do this for you. However you need your phone to be rooted, plus, you will also need to install a custom recovery.
(Xposed Announcement for Android v5/6 + FAQ) http://www.xda-developers.com/xposed-framework-for-android-lollipop-is-here/
(Download Xposed Framework for Android v5/6) http://forum.xda-developers.com/showthread.php?t=3034811
(The actual module, which can only be installed an used once the Xposed Framework is installed and setup properly) http://forum.xda-developers.com/xposed/modules/mod-lockscreen-tweakbox-various-t2843945
However, if you choose to root your phone and go through this solution, remember to take a proper Nandroid backup and a backup of your current "boot.img". This is important and will save you a headache later on, in case something goes wrong during the process, which sometimes does happen, unfortunately.