selected 1 hour interval but it is running once ina day

Can you post screenshot of your task schedule? When you say every hour, does it mean 24 hours in a day?

yes .. because of some reason we  want it to be run every hour,.

And what about your backup plan? Dont you want to backup archived items and indexed data? Scheduled archiving is triggered as per the schedule...if you set it to run contineously for 24 hours then it will be triggered only once in a day...and it is correct. If you like it to trigger multiple times in a day and with less time fram in idle state, then use 15 mins intervals and let it breath for 15-30 mins in between and trigger again.

Note: Symantec recommends backing up archived data and indexed data along with SQL Databases. In 24 hours continue archive schedule you dont have space to backup the data.

Selecting 1 hour or 15 minute intervals just changes the schedule options shown. As the other post says, you should include a pause every so often otherwise the task may not process correctly. 

we are not using journaling licensing and for journal mail box ..using mail forwarding

In that case, and assuming you haven't got a massive backlog to clear down (e.g. in a new EV implementation), then why don't you just have archiving running all the time?  So set the schedule to always run (i.e. colour in the entire schedule).

Then when you backup, make sure whatever process you're using puts EV into backup mode and clears it again.

As tony said you can't have a schedule that runs 24 hours, it will do several passes of the mailboxes and then stop because it's the start of the schedule that loads the mailboxes to scan and so effectively you only start the schedule once and never let it finish

Your best bet is 2 hours with 15 minute stop gap
One hour is too short and I doubt you will keep up with any archiving, but totally agree with rob, it's a bad idea especially as a "journaling" workaround and/or substitution

As JW3 and Rob have already mentioned it will open up a whole world of pain for you potentially.

If the Journaling is being used for compliance then at some stage you or your client will need to access the data and the in built EV Search is not adequate for searching a Journal vault store and exporting potentially thousands of items.

Licence for Journaling, you'll get DA too.  Its there for a reason.