I have been using Hillstone E-Series for four to five years. Now, our company uses the new firewall from Hillstone, with which we are satisfied. I am a customer of the solution.

I have been using Hillstone E-Series for six months.

I have been using the solution for two years. 

We have been using the solution for a year.

I have been working with the solution for three to four months. 

I have been using Hillstone E-Series within the past 12 months.

I've been using it for two to three years.