Reference question please help

For example if you worked with reference two years ago for 3 months but still keep in touch, would you answer that you've known this reference for 24 months or 3 months? It's a question often asked in reference checks and I'm worried that if I put 24 months or 3 months and the reference understands it the other way, then it could affect the job process.
Can anyone advise on this?

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