hmmmm...
1-3 is too soon
11-13 is too late
With Mordred on board I think Morgana will get to know somewhere about 4-7 but I'd like Arthur to know it between 8 and 10

In the middle because if this is the last series then there should be some episodes where Arthur and Gwen starts to doubt Merlin but then realises that Merlin have been protecting them all along

I's say 1-5 is too soon. 6-9 allows Arthur to secretly find out (as I know Bradley and the producers weere quite keen on that idea), giving a realisation period. And then 10+ allows Merlin/Arthur to be confronted with the truth and then allows the build up to the finale.