Depends on what time frame you do the measurement, how you measure it and the type of Arduino, the modern ones have oscillators rather than crystals. I have had a Duemilanove running a system for around 18 months. Part of that is it keeps track of it's millis() versus an NTP aligned computer. It varies slightly with temperature but generally gains around 500 milliseconds per day (~6 ppm). It keeps better time than my Casio watch. A Uno I've tested in the long term is generally within 15 seconds a day.