That's a pretty basic question, so I thought I would ask it in the blog about emini trading basics.
Well, the answer is, as often happens, "it depends." It depends on your target and your market volatility on a given day.
But if you an e-mini futures day trader, you are most likely to be a scalper, and even prefer to go for small, quick scalps. In a case like that your trades should not last long at all, a few minutes on average.
Some of such trades do not take even a minute and I like to call them "quickies" which emphasizes how fast they are. If you are a good KING trader, quickies are what you should encounter relatively frequently. On some days even you may even score a few.
For more about KING, an emini trading course of some renown, check out my site, including the section with its current e-mini day trading results.