According to Grand Ayatollah Sayyid Sistani (may Allah protect him), the distance is approximately 44 kilometers. Source: Minhaj al-Salihin, Vol. 1, Issue 884.
According to Grand Ayatollah Sayyid Khamenei (may Allah protect him), the legal distance (eight farsakhs) that necessitates shortening the prayer is equal to 41 kilometers. Source: Message on Prayer and Fasting, Issue 410. May you be successful in all good.