No problem.
You set you sync interval using the Fetch New Data property in Settings >> Mail, Contacts, Calendars.
If you have Push turned on then all accounts that have push enabled (and their servers support it) will receive mail (almost) as soon as it is received. That works pretty well if you're mail volume is low, or you need to keep a close eye on incoming mail.
You can turn Push on and off for individual accounts using the Advanced options.
Fetch gets mail at a interval. The longest you can set it for is an hour.
However, if you set it to Manual then you will only check for email when you open Mail, or tap on the update icon at the bottom of the mailbox list.
Again, you can set manual mode in individual accounts using the Advanced options.