CallInterval is in seconds.
You can set this to -1 which will bypass waiting between calls. Theoretically there is no upper limit, but you should probably take a look at the Schedule setting if you want a high value.
As the description mentions, this setting is only for polling adapters such as the built in Email, FTP, File and SQL inbound adapters.
It would probably help to understand how and why the call interval is being used.
A polling adapter should try and do two things, yield to its host as often as possible and sleep when there is nothing to do. If it does not yield very often then a production shutdown will not be able to bring down the adapter cleanly. If the adapter never goes to sleep then its going to hammer the CPU and IO 24/7 even when there is nothing to do.
To get a balance between going to sleep and working, the adapter can set a property called %WaitForNextCallInterval. This can immediately re-invoke the adapters on task method. The adapter would normally set this to 0 when its finding items, and to 1 when there are no items left. By setting it to 1 it gives other service a better share of the CPU and IO.
If your using one of the above adapters then this behaviour is built in. Even if you have thousands of items to collect, they will all be collected without any call delay, again, only when there are none left will it sleep.
You can find some more information here...