Tax is paid on 1st January every year, if at that date you have more money than the previous year and if your current balance is at least 1,000$.
Example: You start in 1970 on normal difficulty with a balance of 50,000$, you make a profit off your processors, when 1971 comes around you have 100,000$ on your account. In this case you have to pay (100,000$ - 50,000$) * 0.13 = 6,500$.
Tax Values Edit
The tax rate depends on the difficulty you're playing on: