True, Leontes's jealousy and Othello's jealousy intensify in different ways, but while Leontes's jealousy does manifest suddenly, it also surges with his insanity.
Your suggestion about the possibility of Leontes's jealousy occurring before the events of the play is perhaps unanswerable.
Polixenes mentions events that are foregrounded:
We were (fair Queen)
Two Lads, that thought there was no
more behind,
But such a day tomorrow, as today,
And
to be Boy eternal.
WT (1.2 lines 62-65)
We were as twyn'd lambs that did frisk i' th' sun
(º twinned)
And did bleat the one at th' other: what we
chang'd
Was Innocence, for Innocence: We knew not
The
Doctrine of ill-doing, nor dream'd
of any that did:
WT (1.2 lines 67-71)
Leontes mentions Hermione's wedding vows at lines 101-105. The wedding between Leontes and Hermione could have been at most 23 or 24 years preceding the beginning of the play. This is evidenced by lines 153-156 where Leontes states:
...Looking on the lines
Of my Boys face, me thoughts I did
recoil
Twenty three years...
WT (1.2 lines 153-155)
In the preceding passage, Shakespeare is referring specifically to Leontes's delusion of seeing himself 23 years younger as he looks on the boy. (This idea is echoed in the Sonnets.) It also gives readers a glimpse into events that are foregrounded. The question remains of what Mamillius's age is. If Mamillius is 23 and Leontes is 46, we might presume that Leontes and Hermione were married at least 23 years ago. The difficulty with this reading is that Mamillius is very immature for a 23 year old. It seems more likely his age is about eight years old and Leontes is 31 at the beginning of the play. In this scenario, Leontes is about 23 years old when he and Hermione are married.
As to the progression of Leontes's jealousy and insanity, this instance
of Leontes's mental state (when he hallucinates he is seeing himself younger) occurs after his jealousy is initially triggered.
Later in the same scene, Leontes rants his with a jealous imagination, where previously, he could show a courtly grace:
Is whispering nothing?
Is leaning Cheek to Cheek? is meeting
noses
Kissing with in-side lip? stopping the Career of Laughter,
with a sigh?
(WT 1.2 lines 283-286)
Leontes's insane jealousy increases steadily and quickly causing him to believe facts that cannot have happened. In the Second Act, despite having earlier acknowledged that his son looks exactly like him (it is said of the father and son that they "are almost as alike as Eggs." WT 1.2 lines 129-130), Leontes now believes that that Polixenes is the father. (WT 2.1 lines 60-63)
But to answer your question, the exact moment of the onset of Leontes's jealousy is when he says:
Too hot, too hot:
To mingle friendship far, is mingling
bloods.
I have Tremor Cordis on me.
(WT 1.2 lines 108-110)
Note: The stage direction, "[She gives her hand to Polixenes.]" which precedes the quotation is not in the First Folio.
[The line numbers given are approximate and follow the Bantam edition, and note the quoted text is my edit from a facsimile of the First Folio.]
