Hope this helps

Preposition "by" implies the action is to be performed before the 2 month period elapses. In Mongolian this can be expressed as "хоёр сарын дотор" (literally within two months)

"a couple of" implies uncertainty or approximation. More likely this person will return within 2 months, but still there can be variation like 1 month and 2 or 3 weeks, or 2 moths and 1 or 2 weeks. This uncertainty or approximation can be expressed in Mongolian as "ганц хоёр сар болоод" (literally one or two months) or "хоёр гурван сар болоод" (literally two or three moths)

Is it right?