It’s gradual, but it’s already happening. I was driving through the area this weekend, down Jones Sausage past the new Amazon distribution center, and then along Garner Road, past Main Street and the proposed commuter rail stop and down as far as Vandora Springs. I few things I noticed:
That Amazon distribution center is just colossally massive. The projection is 1,500 new jobs, which I’m sure will accelerate Garner’s growth. It also looks like the site will have its own dedicated bus stop right by the front door, which is great.
There’s are 160 new townhouses getting built near the corner of Garner and Jones Sausage (The Avenue at White Oak). That whole stretch of Garner Road is really going to build up fast once the commuter rail comes in.
I also saw a SFDH home getting rebuilt within walking distance of said train station. 
Main Street is showing signs of life that weren’t visible even a couple of years ago. Again, once the commuter rail is up and running, it should be transformative for Main Street.
Along Timber Road, there are commercial real estate for sale signs all over the place. That’s another corridor that is going to build up like crazy in the very near future, no doubt.
There’s a lot else happening, too. There’s ground broken on Aversboro Road just south of Timber, for what I assume is more housing, but I can’t find any information about it online. But, yeah, it’s already happening.