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You can get that number by using the cli_wallet and get_account..
Next release of piston will have an interest command that looks like this:

+---------+-----------------------+---------------------+---------------+-----------+
| Account | Last Interest Payment |        Next Payment | Interest rate |  Interest |
+---------+-----------------------+---------------------+---------------+-----------+
| clement |   2016-07-05 14:40:48 | 2016-08-04 14:40:48 |         10.0% | 0.872 SBD |
+---------+-----------------------+---------------------+---------------+-----------+