Hey,
Everything is possible. Just from experience though even if you gain 1" erect girth in one year with this work out which translates to 2mm per month accumulated growth approximately,
it is highly unlikely that you will cement this. Meaning that you may gain this 1" but at times even after one year it will fluncuate between 0.5"-0.75". I think if you do manage to get 1" in one year (If you have had newbie gains so far it is possible though), then it might take you 6 more months to actually own it. I have noticed for example even if I gain 2mm in girth it takes me 2-3 months around that point until I see 1 additional mm in girth. It is a very slow process.
However, keep in mind that an increase of 13mm in erect girth is equivalent to 6.5cm in height for an adult male statistically speaking!!!
So every mm counts. I know that if I was 6.5cm taller for example I would have been taller from all my friends! It really makes a difference. So imagine where 2cm in girth increase would put you