Clock starts from date of first exam.
18 months to have passed all 14 (13 for Heli) from the date of the first exam.
Can sit a paper up to four times (ie fail 3 and pass on the 4th go) else you start again.
Have a total of six sittings in which to attempt and pass all 14 (13 H) else you start again.
Thus the most economical route is sit 14 in one sitting and then mop up any 'non passes' in a second or third.
Or sit 7 then 7 within 18 months,
Or 5:5:4
etc