The second round of $600 stimulus payments is already hitting Americans' bank accounts. But with holiday debts to cover and increased utility costs as winter sets in, this money may not last very long. 

For the average working American living paycheck to paycheck, the latest $600 stimulus check will last three to three and a half months, according to an analysis from Earnin, a mobile app that allows you to access your paycheck early. 

Using data from over a million app users, Earnin analyzed how long it will take for bank account balances to return to the average amount they were prior to the federal payments hitting the account. During the first round of $1,200 stimulus checks, Earnin found that it took bank accounts about six and a half months, or 25 weeks, to return to normal fund levels.

Those who use the Earnin app tend to be living paycheck to paycheck and have lower bank balances, says Ram Palaniappan, CEO of Earnin. "Many of them would be people who work retail, nurses who work at hospitals, government employees such as TSA agents and people in call centers," Palaniappan says. "This is a large group of America."

But the latest $900 billion Covid-19 relief package only allocated up to $600 per individual ($1,200 for married couples filing jointly), including dependent children under age 17, if you earned less than $75,000 ($150,000 for couples) in 2019. Stimulus payments start to phase out if you earned more than that, stopping completely for those with adjusted gross incomes of $87,000 or more ($174,000 for married couples). 

Less money means it will take less time to spend it. And lower stimulus payments will hit those living without any savings much harder, Palaniappan tells CNBC Make It.

For Americans with less than $100 in their checking accounts, Earnin estimates that the stimulus check won't last more than six weeks. About 1 in 5 of those struggling financially will spend the money right away, Earnin estimates.

"The $600 stimulus check will last for different amounts of time for different people. But for the people who are even more distressed financially, it's going to be about one and a half months," Palaniappan tells CNBC Make It.

Of course, this is just an initial estimate, Palaniappan says. The timing of these stimulus payments plays a role as well. "It's a time of higher expenses," Palaniappan says. It's just after the December holiday season, which generally means people are paying off holiday gift expenses, experiencing higher energy bills and potentially earning less income due to time off.

Additionally, unemployment is still pretty high. "Let's not forget that it's still really tough for people to get a job," he says. 

Plus, at the beginning of the pandemic, many Americans were able to defer payments on their credit cards, student loans and mortgages. While programs have been extended for some, for others, those debt obligations are starting to catch up, Palaniappan says.

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