Today I asked the question of the 2 Theravada monks whom I am tutoring in English: What is the likelihood that a lay person can become enlightened in their current lifetime? Their answer -- very unlikely; that lay people may live well over 500 lifetimes before enlightenment. That even Siddhartha Buddha underwent hundreds of lives before nibbana. And that they (the monks) are unlikely to reach Buddhahood in this lifetime.
Are they right? I don't know. But it is what they teach.
Also, beliefs on this may vary by which school of Buddhism one is practicing.