I'm assuming that getting the stock will be an ongoing process year after year. If so, I'd likely hold the stock for long enough that I could pay long-term rather than short-term gains. This assumes, of course, that you think it is a company with the long-term future.
If it was a one-shot offer, I'd likely hedge my bets and sell some ASAP and some over time.
Over the long run I'd aim to have no more than 10% of my portfolio in one stock. If you're starting out and your biggest investment is this stock option obviously it would take a while to get to 10%.