Not sure what the bare minimum is but to answer the second question as to why most don't achieve this I think it's down to 2 things:
1. Consistency. You need to train regularly and push for progression over a decent length of time.
2. Dedication. You need to be dedicated enough to think about what you're doing. If you want to gain muscle where do you want that muscle? What exercises will work best for you? If you want to get stronger then look at your technique: how do you move that bar? Are you using all of your muscles to best effect?
With consistency and dedication and enough time, anyone can achieve well above average results in their given discipline regardless of genetics.
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