If collagen takes 3-6 months to form, why only wait 4-6 weeks between Infini RF sessions for acne scars?

The collagen and elastin build-up after each Infini (and the newer version of the device called Genius-- which by the way is a lot less painful) happens over about 4-6 months, after which the fibroblasts go back to a dormant state if you don't continue to stimulate them in some way. Ideally, you will do a repeat treatment when those cells are revved up to their max activity to build on the results. We say 4-6 weeks for the soonest treatment, but in fact you can extend that to 3 months without any detriment-- however-- ideally you don't wait until those cells are slowing down. I always tell my patients to give me a year-- it takes a long time to remodel the dermis-- but Infini / Genius is the best way to do it. 

Great question. You are indeed correct with the 3-4 month rule. It is balance between motivation of the patient and spacing out the treatments. If 6 sessions are required (depending on scar type and expectations of patient) , and spacing is 4 months apart, then revision may take 2 years or longer. Majority of patients would like to complete treatment in 6-12 months, however if you can space sessions out 3-4 months, well, that is ideal from a physiological point of view. Good question!The ideal treatment for acne scars will depend on the morphology of individual scars. For example, rolling scars are best treated with subcision, atrophic scars with fillers, pick-ice pick scars with TCA or Phenol CROSS/ Paint, box-scar scars with fractional laser/ modified TCA/ phenol chemical peels, hypertrophic scars with steroid injections/modulated fractional delivery. 
Energy devices such as lasers, microneedling RF and bipolar radiofrequency are other methods to treat ‘general’ scar types. For example devices can improve rolling-tethered scars as well as ‘deep - superficial scars’ such as broad ice-pick scars and broad box-car scars. Devices should be viewed as a method of scar revision, and rarely as a sole modality. Exceptions do apply- for example in patients with predominantly shallow/broad box-car scars and or superficial pits. 
Another factor that will determine the ideal combination of treatment is the skin type of patients. Skin typing used to determine the ‘skin colour’ of patients. Darker skin patients will require modifications to treatment parameters. This includes a lower percentage of chemical peels, conservative protocols for lasers and energy devices and in some cases pre and post treatment of skin to reduce but not eliminate the chances of PIH- post inflammatory hyperpigmentation- skin darkening. 
The very first step in scar revision is to accurately determine the subset of scars you have. This is best done with a real time examination. Factors such as animation (rather than static photographs) are frequently used to determine the amount of ‘deep scarring’ /anchored scars or tethering. Palpation or touch is also extremely important to determine the presence of dermal and subdermal fibrosis. Scars that have altered ‘texture’ either hard or soft (atrophy) are more challenging to treat compared to scars with ‘normal’ texture. In some cases the use of angled lighting can be employed to accurately map scar types. 
My answers given in the context of this question is a general plan given the limited information. For an accurate guide, one has to take into consideration factors such as animation, touch, feel, real time examination and most importantly subjective measures, namely patient expectations. A real time consult with a procedural dermatologist or plastic surgeon is required for the best accuracy- diagnosis. 
All the best, Dr Davin S. Lim, Dermatologist. Brisbane. Australia.