While abroad, you must enroll in a full-time course load as defined by your program. Since many programs operate on a semester system, credits are converted into the appropriate number of units at Northwestern after your study abroad transcript has been reviewed by the Office of the Registrar. Courses worth less than 2 semester credits are not transferable (see Choosing Courses to Take Abroad for information about other non-transferable courses).
You can earn a maximum of 6 units at Northwestern for one semester abroad, which is the equivalent of 15 semester credits. Furthermore, 6 units at Northwestern is the minimum full-time enrollment for 2 quarters (3 credits/quarter).
Therefore, if you are participating in a program that requires enrollment during 2 quarters (Fall+Winter or Winter+Spring), you must enroll in a minimum of 15 semester credits or the equivalent thereof. If you enroll in fewer credits--or if any of your courses do not transfer back to Northwestern--you will not earn the full 6 credits at Northwestern, and you may encounter serious problems in terms of the Undergraduate Registration Requirement and possibly financial aid.
If you plan to study abroad for the full year and need to earn 12 units of credit at Northwestern (4 units per quarter), you will need to enroll in a minimum of 31 semester credits during the year. 30 semester credits translates to approximately 11 units of credit at Northwestern.
Please refer to the Requirements section of your program page for any specific enrollment requirements. If you have questions, contact your Study Abroad Adviser.