How Much Time Should Be Left Between 1st and 2nd Treatment of Sclerotherapy?

I would wait 3-4 weeks between sclerotherapy treatments.  This will allow enough time for bruising to resolve.  

Sclerotherapy can be done 2-6 weeks apart but usually is done 4 weeks apart and require 2-3 sessions as an average but some stubborn varicosities need 4-6 sessions.  Support hose is very helpful in resolving the varicosities quicker.  Most sessions run $350-450 each and are done with pilodocanol or foam sotradecal.  Hypertonic saline isn't used much anymore.

Usually sclerotherapy is scheduled a few weeks apart, but it totally different areas are done on a future treatment, then it might be appropriate to do the untreated area as early as a couple of weeks after the first.

It may vary among physicians, but in my practice, I recommend my patients return every two-four weeks for their sclerotherapy appointments.  It may take up to 2 weeks, perhaps occasionally longer, for results to be apparent after a sclerotherapy session, therefore you should not be treated more frequently than this. 

The timing between sclerotherapy sessions in the same areas varies among treating vein specialists. In my practice, the time between sessions is at least 3 months, provided that the entire area was completely treated in the first session. This allows enough time for the bruising to resolve and, more importantly, for the treated veins to thrombo-obliterate and heal.