They can send one out at anytime, but after 19 months is probably about right for a 2 year prognosis. (or yours may have been an 18 month prognosis).
The prognosis date is when they say you should next be reassessed.
They set a flag on your DWP computer file to automatically send out a referral to CHDA a few months before that date.
(The computer automatically sets this 'at least' 2 months before the prognosis date they enter, but of course that depends on just what prognosis date they enter).
See the last line of this: https://d.pr/SU9N1l
CHDA get the referal by computer and they send you an ESA50.
The idea is that the 'few months' means your reassessment will be made on about your prognosis date, of course it never works like that and usually takes longer.