Is there a 40 year term life insurance plan available? 

Also, what is the longest term for term life insurance policies?

I don't know whether there is a 40 year term life insurance plan. That would depend on the life insurance company and what they offer. Also, your age and health would impact your ability to qualify for term life insurance for that long of a duration.

Usually, life insurance companies offer level term life insurance with terms of 10, 15, 20, 25 or 30 years. 

With level term, your life insurance premiums stay the same each year throughout the term of your policy, and, the amount of life insurance stays level.

So, if you have a 10 year term policy of $300,000  for $250 annually, you would pay $250 each year and have $300,000 of life insurance coverage for a term of 10 years.

Level term life plans are the most popular type of term insurance policy because it offers the most affordable rates and guarantees level premiums for the entire term of your policy.