Unfortunately no precise length can be stated as sails differ a little in the precise dimensions and type of batten pocket.

If the pocket has elastic tension then you should cut an over-length batten to suit.  Aim to have it only just long enough, without it creating pronounced lines of tension in the cloth.  Some expermentation may be necessary!  An excessive compression load in the top batten will cause it to curve the sail too much, making it difficult for the batten to flick from side to side during a tack or gybe.

If  the batten sticks out of the sail at the leach and small string loop is used for tensioning it is much easier to get it just right.  In this case it may also be adjusted to suit wind strength: loose in light wind, tighter in stronger winds.

Finally, if you have a tapered batten, it is the stiffest end which goes at the leach.