"ATPL courses should last not more than 18 months"
That would appear to be only the case for the ATPL(H) VFR. Notice the difference between paras 1 & 2... There's no similar phrase for aeroplanes as far as I can see... (remember that the AMC is not law, just recommendations)
H. ATP modular course — Helicopters1.
Applicants for an ATPL(H) who complete their theoretical knowledge instruction at a modular course shall hold at least a PPL(H) and complete at least the following hours of instruction
within a period of 18 months:(a)
for applicants holding a PPL(H) issued in accordance with Annex 1 to the Chicago Convention: 550 hours;
(b)
for applicants holding a CPL(H): 300 hours.
2 .Applicants for an ATPL(H)/IR who complete their theoretical knowledge instruction at a modular course shall hold at least a PPL(H) and complete at least the following hours of instruction
a)
for applicants holding a PPL(H): 650 hours;
(b)
for applicants holding a CPL(H): 400 hours;
(c)
for applicants holding an IR(H): 500 hours;
(d)
for applicants holding a CPL(H) and an IR(H): 250 hours.