If you want to keep to the service schedule set by Triumph it's mileage or time which ever comes first so each 6000 miles OR 12 months.
So if you are leaving it 2 years, regardless of mileage, you are not keeping to the schedule.
Whether this is an issue to you is another matter and outside warranty it has no effect.
Bear in mind things like brake fluid and coolant should also be renewed on a regular basis. Again mileage or time to stick to the schedule.