My homocysteine level is 20, 17 and 20 again.the thrombosis unit told me to take b12, b6 and folate (folic acid). for how long and when should i get rechecked.

They are quite right: Get it checked again in 6 months.

Life long: I assume you are reporting your level in μmol/l and in that case your levels are high.If you have a thrombosis related to elevated homocysteine level you should probably on those b vitamins for the rest of your life. You should get your homocysteine level rechecked in 6 months.

B12 is important: B12 is important if one has documented B12 deficiency. B6 should not be taken in large doses; it can contribute to a neuropathy. Regular Folic Acid does nothing for homocysteine excess; some people lack the enzyme needed to activate Folic Acid in the body. One should take methylated folic acid. Metanx contains it...Or one can purchase methylated Folic Acid 800mcg capsules; it might help.

Low hcy: The abnormal homocysteine level increases your risk of clotting -- thus stroke, MI, etc - particularly in context of hi bp. For sure your need for regular intake of sufficient b12, b6, and folate (folic acid) is essential to correct hcy levels. Vit supplements are excellent --- and life long may be necessary --- though diet adjustments may sustain you. I rec at least one - two yrs of specific supplements.