Having attempted something like that in the past, it gets pretty tricky growing the area under the chin. If you're not careful, you can easily cut off too much hair while trimming it, just because it happens to be pointed in a different direction today. Use a comb on it after a shower to see where things really are.
Scissors, a hand mirror, and beard trimmers go a long ways. I'd suggest growing it out until it's a bit long and somewhat unkempt, then trimming it up a bit (leaving some of the new growth), and repeating. You want to let the density fill in over time, but keep the longer crazier hair under control.
It takes awhile.
To have the length and the density to hold the shape at that length, I'd estimate a good year to 18 months.