Need formula to create credit card payoff spreadsheet that accounts for additional monthly charges

I’m looking for a formula that I could program in a spreadsheet in order to create a credit card payoff calculator that accounts for recurrent monthly charges. I’ve been searching for days on Google trying to find one but they all assume that you stop using the card altogether in order to pay it off. The reality is that a lot of people still use the card to pay bills but still want to calculate how long it would take to pay off an outstanding balance on it as they use it.

For example, say I have a $10,000 balance on my card and add $500/month on it (from recurring bills). How can I calculate how long it would take to pay it off given a fixed payment amount and a given annual interest rate.

I have found a web site that allows me to do that, which I would like to reproduce into a spreadsheet and can’t find the formula to do it.

Could anyone help me with that?