The moment of testing all depends of the methodology you used during the implementation of your system.
That means Waterfall, Agile, a hybrid model and Deve0ps will be used in different scenarios. Each project has it’s own rhythm in terms of testing.
But, just make sure you don’t use Waterfall in case you do a new upgrade of your SAP system. Why not? Wim Coremans is explaining when you should test and which methodology you should use in which case in his next video