Hi Zhi. It can be years. We have had patients come to us regarding a condition called "hemosiderin staining" 1-3 years after their vein injections. This condition causes the are around the injection site to "brown" or bruise semi-permanently. The good news is that we do have the ability with q-switched lasers to break up this pigment. Finally, one of the main reasons our olive or dark skinned patients opt for laser treatments rather than injections for their veins is that they are able to avoid the discoloration issue altogether.
In almost 100% of patients, any discoloration in skin following sclerotherapy spontaneously resolves within a year (most will resolve sooner than that). This pigment is the result of hemosiderin (a breakdown product of red blood cells) that has been deposited in the tissue while the body is resorbing the treated blood vessels. Rest assured that this pigment will resolve. One way to minimize the potential for this pigment to occur in the first place is to follow-up with your physician approximately two weeks after your sclerotherapy visit. At that time, any coagulum (clotted blood) that have formed following the injections can be drained by your doctor (thus preventing breakdown products from those red blood cells from being deposited into nearby tissue and causing transient discoloration).