I still have trouble ending ARGs. Most of them have logical endpoints, like the end of expedition, ending the ARG once someone has figured out the last clue, and so on.
Figuring out when to end an ARG can be easy when you have built-in goals for people to accomplish. It can be difficult when you don’t build in measures for success. That’s when you can involve the players. You can ask the players to give some input on when to end an ARG.
You will have to be subtle however, if you choose this option.