I usually place them 2-3" apart. Sometimes even closer. I would place more in the center where light is more intense than the corners and replenish them every 3-4 mos.
I think immediate result depends on some factor like your CO2, dosing, lights, etc. In my tank, I usually see results within 2 weeks.
Should I continue to dose my dry ferts? I don't dose much because I've cut my lighting way back.