Say someone gives me $90k in cash and I put it in my checking account. How much and for how long do i pay tax

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If the money is truly a gift, there is no tax on that money. Depending on the income earned on that money will determine the amount of tax. As long as the income exceeds all your deductions, you will always owe tax. Say you earn 10% (don't you wish), you would have an income of 9000 for the year. A single person with no dependants would then have taxable income of 550. (9000 - std ded 5150 - exemption 3300) Hope this is what you were looking for.