How many hours is a full day of trial versus a half a day of trial?

That's up to the judge, but a full day is usually 9 to 5 with a lunch break, and a half day is a morning or an afternoon.

I Agree with my colleague above. A full day will generally be about 8 hours, while a half day will be approximately 4 hours in either the morning or afternoon.

As Brent writes, the judge has discretion to adjust the hours if needed. And a "full day" can spill over into the following morning if needed.

This truly can vary, and is entirely up the discretion on the judge. Do not plan anything else on a trial day, even if it's only a "half day" trial. The rules of thumb stated by my colleagues are correct -- just be careful not to rely upon them as set in stone due to the judge's ability to do what (s)he wants to time-wise.