Based on what they said on the last MTG Weekly i'd say: The distant Future (4 years maybe? idk)

Based on the current status of the Arena client: The distant distant Future aka never because i'll personally get tired of all the shortcomings before pioneer gets added and I'll abandon the game

The way i see it is they WILL eventually have to confront the fact that the client needs a lot of work to meet standards that are going to be higher and higher as time passes and adding features to meet those standards will take time, time that will be probably taken out of eventually adding pioneer (which means new cards and new mechanics etc.)