How long would it take for a steady state concentration from 100mg of lamictal to 50 mg--keeping in mind i also take 600 mg of tegretol.

Probably 2-3 days: Since the half-life of Lamictal is ordinarily about 33 hours (presuming normal kidney function) and the elimination half-life is decreased significantly by the presence of tegretol that speeds lamictal's metabolism, i think it should only take 2 to 3 days to reach steady state on 50 mg daily after having previously been on 100 mg daily.