Who knows? It isn't just about ratings. The BBC has cancelled successful series before simply because it's felt a show's gone as far as it can. There's also the cost of producing it to consider. Even if it continues to pull in 7/8 million viewers each week, if it's felt the cost of producing it is prohibitive then it might be cancelled.
I think it’s unlikely whilst it’s still successful but you never know.
My rough guess would be that if it dropped below 5mil (after time shift/iPlayer etc.) consistently it might be in trouble.