At some eventual time in the future:
— By and by he'll understand.

synonym: later.

Roget 132: early, soon, anon, betimes, rath^†; eft, eftsoons; ere long, before long, shortly; beforehand; prematurely etc. adj.; precipitately etc. (hastily) 684; too soon; before its time, before one's time; in anticipation; unexpectedly etc. 508.    suddenly etc. (instantaneously) 113; before one can say 'Jack Robinson'," at short notice, extempore; on the spur of the moment, on the spur of the occasion [Bacon]; at once; on the spot, on the instant; at sight; offhand, out of hand; a' vue d'oeil [Fr.]; straight, straightway, straightforth^†; forthwith, incontinently, summarily, immediately, briefly, shortly, quickly, speedily, apace, before the ink is dry, almost immediately, presently at the first opportunity, in no long time, by and by, in a while, directly.    ... show more

Roget 119: at that time, at which time, at that moment, at that instant; then, on that occasion, upon; not now, some other time.    when; whenever, whensoever; upon which, on which occasion; at another time, at a different time, at some other time, at any time; at various times; some one of these days, one of these days, one fine morning; eventually, some day, by and by, sooner or later; some time or other; once upon a time.    ... show more