The question put looks to be a little ambiguous. He seeks to know the period of time to become a spiritual entity. When I plan a journey of thousand miles over a fixed period of time I have to consider factors in mind which includes condition of car, competence of the driver, the much important traffic conditions particularly the road, as also unforeseen conditions..
Each and every chapter of Holy Geeta is a guide for a seeker for his ultimate upliftment. In the context of the present question from among the treasure trove I may like to quote the following slokas of Geeta
Ch 18 sloka - 13 Panchaitani Mahabaho karnani nibodh me. Sankhye kritante proktani sidhhye sarva karmanam!
14- Adhisthanam tatha karta karnam cha prithak vidham, vividhhasch prithak chesta Daivam chaivatra panchmam!! .
Sloka 15 & 16 are equally relevant. These in fact deal with the psychology and mechanism of pursuing the right path of action.
As regards to the period sloka 19 of ch 5 is a direct assurance. If the path is pursued accordingly, attainment is here and now only. One flash of light promptly removes the darkness of a cave which remained in darkness for hundreds of years.