If you are going to spend an hour-and-a-half ~ two hours (or whatever) chanting a mantra over and over again do it right. Don’t waste your time being half-assed about it, or just pretending to do it on the surface.
If you are going to take your time and energy away from your children, spouse, family, friends, passions, and hobbies – don’t waste it spacing out!
The idea that inattentive chanting is better than no chanting at all is dubious. It probably does a lot more harm than good, and keeps a lot of people wasting their precious time. Inattentive chanting will never in a million billion lifetimes grant anyone the goal of the mantra. Period.
You are just wasting time.
If you are dedicating time and energy to chanting a mantra, you owe it to your children, spouse, family, friends, etc. to chant as attentively as you possibly can! You owe it, literally, to all of them to attain the goal of the mantra, so stop sitting there with your hand in a bag of beads daydreaming!
Krishnadās Kavirāja was talking about this when he wrote [CC Ādi 8.16]:
bahu janma kare yadi śravaṇa, kīrtana
tabu ta’ nā pāya kṛṣṇa-pade prema-dhana
You can do kīrtan and śravana for many births
but you will not get the treasure of Krishna prema
It takes more than just reciting a magic formula without even paying attention to it. We have to study Bhāgavatam to understand the sambandha, abhidheya and prayojana – so we can actually comprehend the sound of the names, and thus invoke them without aparādha, which inattention is the very root of.