I've got patches of chest hair that look totally awesome, and my belly is furry too without being 'eww gross, where's the wax kit?' I've got a neat little happy trail up to my navel.

However, my facial hair i just coming in. I have a decent but thin crop of tiny hairs (1/8 inch long max) under my chin and they go up to the bottom of my jaw line without actually cresting over and onto my cheeks. It's driving me nuts because 1) my chin itches all the dang time 2) I'm baby faced with chubby cheeks that I'm trying to exercise off and I'd look a lot better with a bit of fuzz and 3) the mustache hairs look like I've got dirt on my upper lip and I think it looks gross.

I've tried finding articles through google on how long it takes teen boys to actually have some decent hair growth but have failed. So when do boys start to get good growth instead of weird patches? What's an approximate time that trans men who are on T typically see noticeable growth? I've read that allowing the hairs to grow and not shave them for at least 4 weeks can mean you see more growth then what you're used to. I've tried to let my chin hairs go for a week, but the itching got so bad I shaved and had immediate relief. What can I do for itching? I'm pretty sure it's just the hairs curling up and stabbing my skin.

There really is no timeline for when guys get facial hair, it's all genetics. I know one transguy who has next to nothing and he has been on T for many years. I know a cis male who can grow nothing decent but a mustache and he is in his 50's.

As far as itching, that happens to me when I get to a certain length of hair. When I grow a beard I either shampoo my face or shave it off. I can't handle itch.

I agree with Simon.  There's no real answer regarding timelines because everyone's genetics will affect the rate at which they grow hair (if at all) and how much they will grow. 

I've pretty much read that there is a trend with most guys (trans* or those taking T) in that the beard will begin to "fully" grow in (that is, without the patchiness) at around 5 years. 

I'm at 2 years on T and have a fantastic neck beard and mustache, but it's only just creeping up on my cheeks and there's still fuzzy light fluff on my sideburns. 

Sounds like you've got near the same hair pattern I had in my old life.

My chin and neck hair grew thickly and itchy, mustache area not so much, cheeks were a barren wasteland. My Grandpa on my mom's side had a nice thick beard, can't say I've ever noticed much stubble on my dad or his dad.

Hopefully it'll thicken up for ya. I never had much chest hair but I did seem to be gaining a few more strands over the passing of years before so I suppose there's hope.

as everyone else has said, genetics, Darrin and i have been on T roughly the same amount of time, give or take a few days i think, his facial hair is amaziningly impressive! almost like  a walking carpet, and mine (while im happy with it) pales in comparison, just the luck of the draw

I can empathize with you here mate.
But unfortunately you cant really do much but wait.
  I have the same frustrations though, recently its been bugging me even more my lack of facial hair (and how young I look without it), despite me realizing its silly and cant be helped. Just today, the guy delivering my Mums grocery shopping had to ID me! Im 23.
   Im 4 and a half years on T, and only grow a patch of hair below my chin. It is really frustrating and is getting to me a lot more now as Im sick of looking so young, being treated like Im 15 and not having anything substantial to shave when Ive always wanted to have facial hair. But hey, we have to just wait and try our best not to focus on this negative. Theres always many other positives to choose to focus on, and a lot of how we deal with the negative aspects, is beginning with a positive outlook. Find what youre happy about And try not to fixate on what brings you down since it cant be altered.

Like others have mentioned, there are tons of cis guys who can't grow much/any facial hair either. It's not unusual.
On the other hand, enough of those cis guys are willing to pay to get a beard anyway that there is such a thing as a facial hair transplant. It's expensive, but as a last resort option I could see myself doing it someday if T and genetics don't pan out.