If it's set to IMAP, that time interval set by how busy the server is.
If you need your emails immediately (your boss emails you that he wants you in his office "this second" - a dozen times a day), change to POP3 email, set the update time as short as you can and accept the battery hit. But remember - the whole idea of email isn't "it;ll get there immediately", it's "it'll get there eventually". It was designed that way, it wasn't designed for speed. (Initially [way back before we had decided to create those creatures now called dinosaurs], email servers ran about once or twice a day. They dialed up (there was no "internet") the next server in line and passed email both ways. Then they sat, doing important things. If you were having a conversation with someone, you said something on Monday, he got it on Tuesday. If the scheduling was right, he could get his reply sent by Tuesday, otherwise it was sent Wednesday. You got it on Wednesday or Thursday. That's ONE 2-way exchange. These days we expect typing the message to take longer than getting it there.
If you need instant messaging, there are apps using different protocols for that. IRC is one - the delay is rarely more than half a second. Skype is pretty fast too.