A test on scheduled SSIS job

I have create an SSIS package that monitors an FTP and imports incoming data sets as flat files to the SQL server. (BTW, importing directly to an SQL destination is much faster than importing through ADO connection.Guess why?) It is deployed as a scheduled job on the server. The problem is: what should be schedule interval -- given that the job should be run as often as possible while the timespan to run an importing is indefinite.

I was afraid that the scheduled job would be pre-empted when a new schedule starts. That would cause a disaster -- 'cause no one knows what will happen when importing is stopped in the middle. So I conduct a test: I create a 2-min importing job, but schedule it as run every 1 minute. The result is: the next schedule will start only when the current one finishes. Mercy.