It's completely personal but I think spending three weeks in one spot would be mind-numbing. My wife and I take one big point-dumping two and a half week vacation every year, but we generally travel around a bit and try and mix a solid week of relaxation with a week of culture / experiencing local stuff.

Another important factor is your budget. I'm guessing since you want to keep the stay in the Hilton family that you will use points for the room. What about food / day trips, etc?

I'd really look into vacationing in Thailand. You can easily do three weeks there and it's more accommodating to single travelers. The price of everything is very reasonable and it's pretty easy to travel around the country, so you can do a week in Phuket, a week in Koh Samui, and a few days in Bangkok. I'd also suggest, since you have so much time, heading up to Cambodia and doing a couple days at Angkor Wat. There are no Hiltons in Cambodia but you can get a decent place for $50 USD.

Another option is Australia. There's a cluster of hotels around the Brisbane area if you don't want to travel too much, and the beaches there are very nice (especially in Feb.). If you want to travel a bit Hilton has pretty good coverage down-under. And, again, there's loads to see, the prices are reasonable (compared to USD), and you're doing something wrong if you can't have a load of fun there as a single guy or gal.