I took one 25mg lamictal last night and want to know how long until it will be out of my system?

Don't oversimplify: The simple answer to how fast Lamotrigine gets cleared may be not the whole story. Half of a single dose is gone in 33 hours, and half of the remainder in another 33 hours and so on. But when other drugs are on board such as valproate, the half life goes up to 48 hrs. On top of that, chronic users should not stop at once. Most say taper over two weeks or so.