When you expect an environment of rising implied volatility. A collapse of implied volatility or a rapidly trending market can potentially cause a loss to this position. The performance graph of a time spread at expiration date of the short option takes on the appearance of a tent. Prior to expiration it has the appearance of a gently sloping hill. Holding time spreads within the last 3 weeks prior to expiration is risky since a move in the market can cause the short option to lose more than the profit on the long option.
Time spreads possess both limited profit and limited loss.
Maximum profit is earned if the futures contract in the near month winds up on expiration day at the strike price of the short option. There are futures contracts for each month in Crude Oil, so the Dec and Jan calls relate to their respective underlying contracts. The short Dec 32 call decays more rapidly than the long Jan 31 call because it's closer to expiring and further out of the money. The position then becomes a Jan bull call spread. That's what makes this position slightly bearish.