This is relatively subjective, so I'm going to offer another answer that might be more understandable to you, depending on how you learn best. There's a few different ways to do this, and it depends on what your rhythmic base is - i.e. how many steps of a phrase you are considering a "quarter note" based on whatever notation you might be transcribing.
The basic gist is that a note in LSDJ with no envelope information (with x8 as the envelope perameter, either with an Ex8 command or x8 as the Envelope parameter for the relevant instrument) will sustain indefinitely, until it is either cut off with a new E command, a K (kill) command, or superseded by a new note.
So if you're assuming that a quarter note is equal to four steps on the phrase screen (which is pretty common), you want your note to last 6 steps and you should put a K00 command or another note (or an Ex1 command if you want to get fancy) on the 7th step.