The Collaborative International Dictionary of English v.0.48:

Interminably \In*ter"mi*na*bly\, adv.
   Without end or limit.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interminably
    adv 1: all the time; seemingly without stopping; "a theological
           student with whom I argued interminably"; "her nagging
           went on endlessly" [syn: interminably, endlessly]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "interminably":
   ad infinitum, again and again, and night, at a stretch,
   boundlessly, ceaselessly, connectedly, constantly, continually,
   continuously, cumulatively, cyclically, day after day, endlessly,
   enduringly, eternally, everlastingly, for an age, for life,
   for long, forever, hour after hour, illimitably, immeasurably,
   immensely, in perpetuity, incalculably, incessantly,
   incomprehensibly, indestructibly, infinitely, innumerably,
   limitlessly, long, long ago, long since, measurelessly,
   monotonously, month after month, morning, never-endingly, noon,
   on a stretch, on and on, on end, perdurably, perennially,
   permanently, perpetually, persistently, protractedly, repetitively,
   round the clock, since time began, steadily, the livelong day,
   till doomsday, time immemorial, time without end, to infinity,
   together, unbrokenly, unceasingly, undyingly, unendingly,
   unintermittently, uninterruptedly, unrelievedly, without a break,
   without cease, without end, without limit, without stopping,
   world without end, year after year