When will space in undo tablespace be released ?

Recenlty, our system reports error about snapshot too old during peak hours.  I found a standard Oracle Report will use up 130 M on undo tablespace.   That oracle report usually complete in 1 minute.  

Is it the 130 M space will be released after 1 minute or have to wait for 8 hours ( rentention period parameter) ?

If 50 reports submitted, 6G will be consumed.   I wonder if adding space is the only solution to it.   

Regards,

Christine