1842 to 1854
Sree Sai Baba is deemed to have spent 12 years with his Guru. No one knows for certainty who was his Guru.
It was perhaps a Sufi Fakir or Venkusha alias Gopal Rao Deshmukh. Some say that Baba spent 12 years, under the Margosa (neem) tree in Shirdi, in an underground cave, near his Guru’s Samadhi.
Reference: Satcharita Chp.4, ovi 129 and Das Ganu’s ‘Bhakta Leelamrut’ Chp.26