I have been using the product for a year. 

I have been using this solution for 20 years. 

I have been using the solution for more than 15 years.

I have been using Siebel CRM for three years.

I joined my company recently, but they have been using Siebel CRM since 2009.

I've used the solution since 2013 or 2015. I've used it for a while at this point. It's been almost ten years. 

I have been using the solution for twelve years.

I have been using this solution for seven years. 

We have been using this solution for more than 15 years. 

I've been using the solution for 20 years. 

I've been using it for 10 years.

I've been using the solution for over a decade. It's been 11 years or so now.

I have used Siebel for over 6 years before moving on to Salesforce.