Enemy blood tethers can last forever on the user (potentially also minions - I saw a few minions ticking for damage while out of combat yesterday). I continued to lose ~4 hp per second with around base regen, persisting through map changes and even death (died in town, message stating due to Blood Tether). Logging out fixed the issue.
Noticed this on my mage after doing the study in time quest and progressing through the final areas. I used a TP to return to one of the zones containing blood tether enemies, and potentially left the zone while enemies/blood tether was still alive. The issue could have started earlier though, since it was hard to notice the low damage/sec on my ward while teleporting.