[fpgaminer]

If you need a rule of thumb, for example you want to have a rough idea what your taxes would be like to see if you're comfortable doing some liquidation or something, then sure. I'd back-of-the-napkin long-term gains at 20%, and short-term at 35% (covers both federal and state; assumes U.S.).*

Again, very rough. You need concrete numbers once you do actual trading, because in the U.S. you are required to make estimated tax payments. The actual numbers can, for example, vary wildly from nearly 0% to upwards of 50%.

* These are based on a typical 15% federal tax on long-term gains and 5% for state taxes; or 35% being a comfortable average for most people's effective tax rate in the U.S. for short term gains. The 50% tax rate is approached when you have largish gains, toward $1 mil or so, and live in expensive states like New York or California.