If I am trying to become pregnant when should I stop using 2% Minoxidil?

You need to discuss this with your Ob/Gyn doctor.  There is no absolute rule. Minoxidil is a Category C drug.  Which means:  "Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks."Many women take Category C drugs during pregnancy.

Minoxidil must never be used during pregnancy or by women trying to conceive. Minoxidil does get absorbed and years of experience with oral minoxidil have taught us it can harm a developing fetus. Women trying to conceive should not use minoxidil. 

You can read about the side effects of minoxidil.  There is no report that minoxidil will effect pregnancy, however it is always safe to consult with your doctor about any medications that you are taking.