How many letters does the top answer for “from that time on” have?

The most common answer is thenceforth (11 letters).

Crossword clue: from that time on

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

thenceforth 11 letters wn-def

from that time on

thereafter 10 letters wn-def

from that time on

an amount of time 50% similar → period

at a previous time 50% similar → erst, erstwhile, formerly

at an inconvenient time 50% similar → inopportunely, malapropos

at an opportune time 50% similar → opportunely, seasonably, timely

for a limitless time 50% similar → eternally, everlastingly, evermore

for a short time 50% similar → awhile, briefly, shortly

for the most time 50% similar → longest

one by one; one at a time 50% similar → singly