It depends on the field and on the size of the research group.
Personally, once you reach critical mass of people, I believe the ideal would be for 1/2 yrs to co-author along with 3/4y first authors. One paper/year per 3/4 year phd candidate should be enough to make everyone leave with a nice CV.
Of course, some students might do better (MS research experience) and start "first authoring" before, some might get stumped and start after that, but it would be best to finish the phd with at least 2-3 papers as first author... the competition for positions is only getting worse, both in academia and out...