What is a perimenopause length usually?

Varies widely. Average would be about 8 years. Some patients however, never have the first symptom of menopause during the entire transition. So it may be hard to really pinpoint the onset of this timeframe. Subtly things like mood changes, headaches, pre-cycle bloating, night sweats, hot flashes, may all be indicative of this change. Cycle shortening to 26 days or shorter is also very characteristic.