How quick do these grow?
Its not exact as all planting aspects can vary slightly, but on average you should expect somewhere in in the region of 30 to 40cm per year.
New Zealand privet what heights can i expect and how many plant per metre?
These are already well established plants so two to three per metre, no more than three. Maximum height left untrimmed can be 6 plus.
Are they easily trimmed?
Yes like all hedges you can literally chop off at the required height.
When should they be triimmed?
Early spring after winter and early autumn before winter.