I don't knowthat there is a designated length of time that it's safe to use DXM daily, in concert with kratom, simply because there is not any medical literature investigating this. I do know that there is a decent amount of research in the medical community on DXM being used as an adjunct to opioid treatment, for neuropathic pain, and demonstrating possible anti-inflammatory effects. Personally, I think there is a lot of untapped potential in DXM for pain management.
My experience:
I use kratom daily and have for going on four years now. I use kratom for diagnosed chronic pain due to congential malformations of the spine, which is causing progressive degeneration and increasing pain.
The past 3 years I've been consistently consuming 28 - 40 grams/day of red bali/borneo/sumatra in 2 - 3 doses of 10 - 15g per dose; high quality plain leaf. It has kept me functional and able to maintain a productive life. I began using DXM about two years ago hoping for added relief due to (possible) anti-inflammatory properties and potentiation of kratom. IME DXM absolutely potentiates kratom, as well as synergizes nicely. My experience is that, over time, kratom without DXM was not as enjoyable or powerful and has led me to maintain daily DXM use along with kratom. I take 30 - 90mg DXM per day in a single dose in the evening. I do not experience withdrawal from the DXM if I do skip a day, and I have taken up to two weeks off the DXM as I too am concerned about it's negative health effects. However, I have not encountered any negative health effects that I am aware of and continue this regimen daily. The nausea can be overwhelming though. I have an Rx for ondansetron or I would not be able to maintain the kratom doses; I've experimented without ondansetron and I vomit every time.
That's just my anecodtal experience