Length then girth is the general thinking. But you will gain a little of the other as you try and focus on one dimension.
Which do you want more? Go for that.
Or if you find you are gaining length ‘easily’ then why stop the gains.
Also girth gains tend to be more difficult to acquire. So you may want to start early. Or try and incorporate some girth exercises along side your length routine.
Basically, it’s up to you! (What a rubbish advice giver am I? :) )
The eventual maximum? When you give up… or when growth becomes so glacially slow that there is no point in continuing and you’ve exhausted all the techniques and exercises you are willing to try. There are rare 2”+ length gainers for example, so perhaps anything is possible.
Good luck in your PE.