A trade in crude by a high-frequency quant fund might last literally less than one second. So anything between .05 seconds and 5 years will work... No reason a swing trade won't also. If you're brand new to commodities then I humbly suggest you start with the mini contracts. Maybe the mini gasoline or heating oil. Watch out for Friday afternoons and Wednesday reports. Volatility at any point can explode into heights you may not expect in stocks for example. I've had success so far day/swing trading calendar spreads, esp in RBOB and heating oil, though liquidity dries up sometimes at night. Initial margins are $540 right now, but you may want to hold three times that for example. Personally I hold about nine times that but maybe I'm just paranoid... You can swing trade calendar spreads in CL also. These also greatly reduce margins. Also you might consider the WTI-Brent spread, or crack spreads maybe using minis. These are more complicated but they're also I think better bets, generally less dangerous, and they'll force you to learn something about the petroleum complex and the nuances of futures.
the Committment of Traders (COT) info for the crude showed a net decrease in LONGS as of 06/16/09 http://www.321energy.com/cots.php#067651
why so long term? could not get it! if you have millions of dollars, I suggest you do that. if you just have a $25k account, just strickly do day trading, otherwise crude's volatility will easily kill your account (margin call)
We've taken some crude trades that have last for months. A lot of real wealth in trading comes from position and swing trading.