That depends on how much you trust the media that the backup goes to. Theoretically there is no limit, but then you may also want to consider the amount of time it would take to fully recover from that backup, should the need arise. If you run weekly fulls, worst case scenario would be a restore from the full backup plus restoring from the 6 incremental backups since that full. Now take this scenario out 6 months... Now your restore would consist of restoring the full backup from 6 months ago, PLUS restoring from every incremental backup between that full and the current point in time. Now add to that, if there is a problem with one of the tapes from two months back, it will break your capability to recover to a consistent image. Granted you can use the "restore by job" feature to recover from other incrementals after that corrupt tape, but how much will your users/customers like to wait for that process to complete?
Thought this would be a good point to consider.
Ernst F. Graeler
Senior Engineer III