When describing a short amount of time, there are numerous synonyms for "not long." "Briefly" suggests a short duration, while "momentarily" implies an even shorter period of time. "Shortly" implies a brief time frame in the near future, while "in a little while" suggests a relatively short wait. "Fleeting" suggests something that passes quickly, and "transient" implies a short-lived experience or moment. "Ephemeral" suggests something that is brief or fleeting, and "momentary" implies something that happens for a very short time. "Fleeting" is another option for a brief or passing moment, and "transitory" implies something that is temporary or fleeting. Overall, there are many ways to describe something that is not long in duration.