Synonyms for Interminable:

adj.

continued (adjective)
sustained, unceasing, persistent, persevering, protracted, prolonged, abiding, maintained, extended, continued.
continuing (adjective)
unending.
continuous (adjective)
level, gapless, lasting, repetitive, robot-like, unvarying, consecutive, recurrent, unchanging, repeated, uniform, undying, indefatigable, undeviating, steady, persistent, nonstop, monolithic, flat, unchangeable, habitual, continuing, sequential, regular, unceasing, unbroken, everlasting, incessant, consistent, ceaseless, seamless, straight, perpetual, coherent, featureless, monotonous, connected, invariable, even, constant, un-intermittent, continual, unending, endless, perennial, continuous, homogeneous, unremitting, cyclical.
durable (adjective)
long, livelong, lifelong, durable, perennial, enduring, perpetual, lifetime, long-life, lasting, everlasting, lengthy, incessant, prolonged, eternal.
extended (adjective)
lengthy.
infinite (adjective)
immemorial, innumerable, countless, incessant, immeasurable, numberless, endless, perpetual, protracted, boundless, ceaseless, illimitable, wearisome, long, bottomless, continuous, uninterrupted, infinite, incalculable, Termless, inexhaustible, permanent, constant, long-drawn-out, unbounded, unceasing, everlasting, eternal, unlimited, dull, timeless, limitless.
never-ending (adjective)
undying, ceaseless.
perpetual (adjective)
constant, continual, unceasing, infinite, endless, lasting, uninterrupted, enduring, permanent, incessant, eternal, everlasting, ceaseless, immortal.

n.

long-drawn-out, forever, lingering, day in day out. ongoing, continue, round the clock, relentless, around the clock. unending (noun)
dragging on and on, wearisome, Overlong, dull.

Other synonyms:

Overlong, lingering, long-drawn-out, around the clock, round the clock. ongoing, forever. relentless. Other relevant words:
Overlong, dull, relentless, lingering, ongoing, round the clock, long-drawn-out, wearisome, forever, ceaseless, continue.

Usage examples for interminable

  1. Miss Schuyler listened for what appeared an interminable time, and wondered afterwards that she had borne the tension without a sign. – The Cattle-Baron's Daughter by Harold Bindloss
  2. After what appeared to Hallam an interminable time the doctor came downstairs. – The Stronger Influence by F.E. Mills Young