My dad is 65 and has a slow growing tumour on the floor of his mouth. he's had oral cancer 5x & no more treatment is possible. how long does he have?

Hard to say but...: This is impossible for anyone to know, esp. Since you have not told us the type of tumour he has (there are mult. Possibilities). But even if we knew it is hard to predict. I question whether no treatment is possible. Get second (; 3rd) opinions. Even if conventional options are exhausted alternative therapies- diet, herbs, homeopathy, acupuncture, mind-body approaches etc. Could extend his life.

Impossible to say: It depends on the specific cell type of the tumor and how aggressive it is.

No Physician Knows: It is impossible to make accurate, reliable predictions for any individual, at any time. Occasionally physicians suggest time frames based on loosely defined averages of groups of people, but even a physician very familiar with your father's situation cannot predict with much accuracy. As embodied souls with a mind, complex+fragile brain running a complex+fragile body with millions of orders/sec:.

There's no telling: It could be tomorrow. It could be years. Enjoy each day with him.

Hard to tell: If the tumor is not that aggressive (slow growing) and he maintains the ability to swallow and breath for a while, he may be able to do quite well for a while. It really depends on the type of the tumor, its growth and the involvement of the adjacent organs.