AFAIK, there is no option to kill a job after it has run x amount of time.
I suppose that you could script something up using BEMCMD to check for an active job and then cancelling that, but you may very well wind up with a useless backuop if it is cancelled
again, if you have the disk space availble, can you dump the database(s) to disk and then grab that with a flat file backup, or while they are dis-mounted copy them and backup the copy?