Plant them about 11/2 -2ft apart, let them grow to about 18"-2ft higher than you want the hedge, then cut them back to the height you want, this will give a bushier hedge, which you can trim annually. derekm

That would be good for a hedge about 4 ft. tall. I would plant them farther apart for a taller hedge--rule of thumb: as far apart as 1/2 of the planned height, and expect the hedge to be that deep (distance from front to back) at the base. Further tips: trim it so the base is slightly wider than the top, to keep the hedge full to the base; and never, ever let it get out of control. It never looks right again if you have to chop it back by yards to bring it back into size.

A warning from me - you have to be prepared to prune them at regularly because they grow a huge amount in just one year and if left too long the inner branches die off and will not regrow.

Yes, all respect to Derekm, but I would expect a 'Leylandii' hedge to need pruning several times a year, to keep it under control.