Yes, they will come back, you can pretty much count on it.
I had a clean grow room until I picked up a bag of soil from a local garden center, which was infested with the little bastards.
15+ years later, and after going hydro, I still have the motherfuckers.
The good news is that after all these years I understand how to live with them.
It really is hype that they will destroy your root system, because they can't, there are too many new roots developing in such a short period of time, they don't have the capability to do any real damage.
Where they will fuck you is with a very young plant, one that is around 2 weeks old in moist soil. The larvae will hang out where the stem leaves the soil and eat at the soft tissue there, and basically cut you plant in half.
I've lost more than one plant that looked great at 3 weeks, only too wake up one day and the fuckers 1/2 dead because when it was a young plant it had been attacked at it's base, just under the soil, and damaged.
Once the stem is around a 1/4" in diameter, the gnat's can't hurt the plant, then they are just annoying.
So, I place yellow stickey traps around my seedlings, make sure I have a fan blowing thru the plants and make sure you don't overwater (gnats love wet soil)
There just a part of life if you grow in soil, but basically if you take a few simple precautions, their harmless.
Oh, and as far as you plant producing bud, most Sativas are 10 weeks minimum, and 12 is more like it.
It's a very long grow with a dominant Sativa strain, so be patient.
Good luck
