Soldato
You can't, you can only have it on or off (in account settings) - it'll adjust sync times depending on how often you get emails - the more it pulls in every time it syncs, the more often it will sync (it will decide that you're popular and need to read your mails more) - but on the flip side, if it syncs ever hour and gets nothing for two attempts, it'll increase the interval as you're obviously a billy-no-mates lol
You can turn off sync with a toggle in the pull-down window so you can do it manually if you prefer (that's how I do it)
This is how Gmail works when it pulls mail from other linked accounts e.g. your ISP email etc to your Gmail account. When it is in Gmail, the emails are pushed straight away.