how often to run reorganize index & update statistic maintenance task on sql server 2012?
You should intelligently do index reorg and rebuilds. As a general (widely accepted) rule of thumb is 

If your indexes are fragmented :


If index has less than 1000 pages - do not perform any index maintenance operation.
Less than 10% - then do nothing
between 10% and 30% - then do a reorg
more than 30% - then rebuild them.


I would highly recommend to use Ola's maintenance solution (index and checkdb)

Since your databases are 1GB to max 10GB, depending on your server spec it should not take very long time.