No, this timing is quite generous; you simply won't need that long to explore Qasr Al Watan; we'd allow 1.5 hours. If the weather is not too hot, you can explore on foot the neighboring Emirates Palace, the Founders Memorial, and surrounding hotels such as the Conrad and the Observation Deck at 300. As long as you're back to the coach pick up point at Qasr Al Watan by 6:00 PM. Alternatively, explore elsewhere in town, and get back to Louvre Abu Dhabi by 6:35 PM for your return journey to Dubai.