In many cases the J1 visa holder may not know the duration of his stay in the US. This can pose a problem for purchasing J1 Visa health insurance. Also sometimes the J1 visa holder may be reluctant to pay the entire cost of the for the full duration of the stay at one time.
In both these cases it is preferable to purchase a renewable J1 visa insurance where the plan can be renewed monthly and payments can be made on a monthly basis. Make sure you check the renewability of the J1 visa insurance you purchase to make use of the renewal option. Some plans may charge a small fee of $5 per renewal.