It depends on the condition of your grandfather's immune system and how much his body can endure. Try to make certain he's comfortable. So sorry to hear that.
How Long Does Someone Have To Live If They Have Bowel Cancer That Is To Far Gone To Be Treated?
Only God knows the answer to this question. He will be ministering to your Grandfather, His beloved Child, until the moment he receives him Home. You are in my prayers, and You have my empathy. Bring your Grandad plenty of love in his departing day's. 
You can never put a time on someones life