I'd be nervous sitting on that much, but I'm not honestly sure how likely it is to do something that isn't in line with the broader market. Part of the joy of index investing is not having to know.
If you're asking because you're concerned you're overconcentrated, I'd take steps to reduce that. If you acquired this over time, you could sell the shares that have the least appreciation to bring yourself down to a quantity that fits your risk tolerance. Make such a sale each year on your way to retirement, then convert the remaining shares with the most gains once your income is low to avoid higher capital gains taxes. Given the current state of the market you'll almost certainly have gains, but you might get a good amount out now without too much tax.
But it's really about your risk tolerance. If you can tolerate whatever risk there is and don't want to pay those taxes, you could just hold on to the shares. Who knows, it could outperform or crater. But if it isn't keeping you up at night, what you stand to gain in tax avoidance might be more than what you lose even if it slightly underperforms. If any of us could predict the future we wouldn't need to diversify our portfolios... but alas, we just get to deal with our own tolerance for risk.
