How often should I run data collection

Hi,

We have EV 10.0.3 on Windows cluster and we are archiving Exchange mailboxes and journal mailboxes.

Our strategy to back EV is to do daily Full parttion backups for open partitions .. We have seen that the backup speed is rediculously slow .. less than 2MBps. The data collection is enabled but it is scheduled to run for data older than 10 days.. 

I know that data collection helps in getting backup done faster but would like to know what is the optimal use of it. Would the email retrival become slow in case I start collecting data one day older so I have almost all data collected in cab files.

Thanks