I'm sorry but such a feature doesn't exist. In case your account has a volume limit, JDownloader will handle this automatic and there is nothing for you to do.
In case you want to limit the volume because of your ISP, then I suggest to write an Eventscripter script that monitors the downloaded files /traffic and stops downloading when threshold is reached. You should contact mgpai for this, he is very talented with the Eventscripter. See
https://board.jdownloader.org/showthread.php?t=70525
for some scripts. Maybe there exists already one that does what you want