This is completely dependent on your citation style. Two examples:
For APA, if there are only two authors, cite both each time; if there are between three and five, cite them all the first time then cite as "First et al., 2014" in subsequent citations; if there are six or more, use "et al." every time.
For IEEE, use of "et al." begins at three authors, and you use "et al.".
As you can see there are wild variations. Check which style you're supposed to use, and check how multiple authors should be cited with that style. As a general rule of thumb, maybe use "et al." when there are too many authors.
As another user pointed out it's also dependent on your field, and it's even possible that author names shouldn't be cited in every reference (just a numeric reference like ) -- I just checked a math journal at random and this was like that.
PS: If you're writing something in LaTeX and are using biblatex, I want to advertise the commands
\footcite. They are able to automate most of this (for example with the APA style, they correctly detect which citation is the first).