Spider veins generally worsen with age.
Treatments are very effective, but new veins develop as you get older.
You may be someone with a few in some areas or someone who has them every where and have not worn shorts or skirts as a result. Therefore it really depends on severity.
Depending on how significant or bad they looked when you started treatment, you may need treatment once a year to once in three years. The average would be every 2 years.