What is a Partner Designate?
Partner designates only really exist in relation to external recruitment; internally you would probably say a senior associate was on track for partnership rather than being a partner designate. The level of post-qualification experience necessary to be considered as partner designate is generally 6/7 years plus.
For the purposes of external recruitment, such an individual can be recruited as a true partner designate and often given a timeline to become a partner. This is typically 1-2 years.