What percentage of my total internet service and cell phone bill can I deduct for my home business?

Has it changed for 2018?

It depends. You need to calculate how much you use your cellphone and internet for business and how much you used for personal use. 

For example, lets say your cell phone bill was $500.00 (used 100% of the time). You calculated that you used your phone 30% of the time for business and 70% of the time for personal. So $500.00 multiplied by 30%= $150.00. You can write off $150.00 as a business expense, and $350.00 would be a personal expense. 

If I work from home, 100% of the time, and use the internet for work the entire time that I work, but also use the internet for personal use when not working, how would I determine the percentage of the cost of the internet service to deduct?  It sounds really subjective.

It can be subjective, but the 'default' is the percentage of your Home Office.  So if your qualified home office is 10% of your home, you can deduct 10% of your utilities, including internet.