Paramhansa Yogananda once said to Durga Ma that he was Arjuna in one of his previous incarnations! If his soul took so long to achieve union with the Divine, how long will us mortals take to achieve such union?
Yogananda was, as far as I understand, already an avatar, fully united with God, when he incarnated as Arjuna. He acted the role of the humble disciple, just as he did in the role of Yogananda. Arjuna and Krishna were, as the Indian scriptures say, Nara and Narayana in their previous existence. Then too, I believe, he (Nara) was already one with God. They are described as avatar-twins of Vishnu.
Now concerning your more immediate question: how long will it take us “mortals” to achieve union with God? Yogananda affirmed that it is possible for each of us in this lifetime, if we think that it is possible — and if we really want liberation, if we work on it sincerely, offering ourselves to the Divine with all our heart, all our mind, all our soul, and all our strength. Regular sadhana is needed. The highway to God is the spine, and Kriya Yoga takes you right there. It is at the airplane route to Him, Yogananda affirms.
Such a technique, united with devotion, works like mathematics, it cannot fail: in this very lifetime. But the heart must yearn for it deeply, and the mind must cooperate: it must believe in it. Yogananda taught:
“The simple thought that we are not free is what keeps us from being free. If we could break even that one idea, we would go into samadhi. Samadhi is not something we have to acquire. We have it already. Just think: Eternally we have been with God; for a short time we are in delusion; then again we are free in Him forever!”
All the best for your God-union,