I have been using this solution for three years.

I have been using this solution for four or five years.

I have been working with the product for three years. 

I have been working with this solution for about four years.

I have been using Avolution ABACUS for five years. I use Avolution ABACUS Version 8, though Avolution ABACUS Version 10 is the latest version of the tool.

I've been using ABACUS for two years. I've used it within the last year.

I've been using Avolution ABACUS for 10 years. I used it in my previous company, and in my current company, I'm trying to get it, but I'm still waiting on budget approval.

We have been using Abacus for one-and-a-half years.  

I have been using the solution for approximately three and a half years.

Since 2015.