Your withholdings are percents of your salary. If you don't work at all, and earn no money, you owe no taxes (of course, you have no money, either). But here's how you'd figure out what your gross earning are given that your taxes and FICA are $493.68/month
FICA + Tax = 7.65% + 9.9% = 17.55% = 0.1755 (decimal equivalent of 17.55%)
(Monthly Gross Earnings)*(0.1755) = $493.68
Solve for Monthly Gross Earnings. To determine the number of hours needed to earn those Gross earnings, divide by the hourly rate:
Hours per month = (Monthly Gross Earnings)/($10.65)