The regular period of time during which congress conducts its business is called a

Session

Explanation:

In one year, congress will have the maximum of two sessions. One session could generally made up of one year worth of work for the congress. But, there are several occasions when the congress unable to do as much job as it targeted to within that one session. The second session each year would serve as additional time period when they can do the job that they can't finished within the first session