It will be in temper as long as it's fluid. But the problem is that it won't be fluid very long. Tempered chocolate sets quickly, which is part of the point, but it makes it a pain to work with. Once it has cooled down enough to be solid, you have to re-melt and re-temper.
The best way, of course, is with a tempering machine, which keeps the chocolate at the perfect temperature. If you don't want to blow a few hundred bucks on a very specialized device, you can try keeping your piping bag in a cooler with a heating pad (on the lowest setting) between uses. Or keep it in a warm-water bath, keeping an eye on the temperature (88°F, plus or minus a very small number).
You may have better luck with couverture chocolate, which is a bit more forgiving, and gives very sharp results. That's real chocolate, with a higher cocoa butter fraction. Or you could use the cheap decorator chocolatesque wafers, which set beautifully and taste like wax.