Everybody’s potential is different, so no telling how much you can grow. But some guys have gained 2 full inches in length. 3/4s of an inch in girth is also doable. Both will take a pretty long while unless you are exceptionally fortunate.
I think the two biggest issues to deal with are determination and consistency. Generally speaking this is the thing most guys lack when it comes to reaching their original goals.
Determination to achieve your goals is paramount because you might have to try a few different techniques and possibly several program variations for at least a few months each in order to see what works. Most guys quit after a while with no or disappointing results while they could have taken it as data about what not to do and to help guide what else might work better. Generally you have to give a protocol a try for several months, which is kind of the minimum to see if it is working, and sometimes you gain nothing. If you are determined you change it up and keep trying.
Another bit along the determination line is that once the newbie gains end (which is different for everyone but inevitable) then you have to work harder and smarter for every incremental gain. This gets harder to do once you cross a certain size threshold. For most guys, once they reach 8+ inches (sometimes after years of work) the effort to get another half inch can feel less worthwhile, especially if you are getting feedback from partners about how big you already are.
Consistency is the other pitfall that prevents most guys from getting to their original goal. No matter what, gaining 1 inch or more in length and 1/2 inch or more in girth is a LONG journey. Months can go by with little or no gains. Sometimes you have to take a decon break to get the gains started again. Sometimes you need to take a break take one to get your head back in the game. The day-to-day grind loses its luster pretty quick, especially once the gains slow down. But if you want to get all the way there, you just have to log the miles and keep the faith.