You know... speaking as someone who wanted to be there more often than I could... but with terminal cancer. Its not going to likely be all that long. If you leave before you might live to regret not having those final months with that parent. When they are gone they are gone. And towards the end they will in all likelihood be discussing things they never told you your entire life.
Speaking as someone who lost his father to cancer. The best I was able to do was weekends due to te distance I lived. I would have given anything to have been able to be there more than I was. Particularly in hindsight.