Usually after about 2-3 months I'll let guys (and girls) start to spar, but it's situational. First few times they "spar" it's just jab round.
45 seconds one guy jabs, the other guy defends.
Then vice versa.
Then the last 30 seconds they can both jab.
I find it helps ease them into sparring this way. They can focus on one thing at a time either attacking or defending (plus footwork, which is obviously very important). I can also tell how they handle pressure this way. Also, since it's just the jab, there's very little chance of anyone getting hurt their first few times and turning them off from sparring.
Some guys might do this for weeks, other guys are fine and I'll add a bit more each session. Maybe just jab and cross. Or just KK Sparring (body punches shots only), etc.
It seems to be working pretty well and the students get a chance to apply the techniques they've learned in a real situation much sooner then waiting 6 months or so. This way they can really see how everything works right off the bat and take the technique/pad drills more seriously.