The module itself does not state explicitly what level ToH was meant for, however, we can estimate based on a few factors to come up with a reasonable guess.
The original White Plume Mountain was 5-8 and it is now recommended to start it at 8th. Against the Giants was 8-12 and it is now recommended to start it at 11th.
P5 of the book also indicates that it is possible to run these as a "super campaign" even though they were never written to do so and would require some leg work on the part of the DM to connect them in some logical way.
What this all means is that with the starting of 10th recommended by the original and "IF" you combine them to a super campaign you would start ToH right after AtG, which would estimate about 14th or 15th. This would be an estimate on my experience with how converting some of the older modules and the XP earned in them has worked for me in the past and by what they have done to the modules from previous editions. I own all the originals so I can see where things have changed and have played every edition so the estimate should be pretty close.
(I actually had a DM start us there in the original ToH at level 3, citing that since it was a "thinking person's dungeon" if we were smart we could get through it).