If you bought the time share for personal use, then just as in the case of a sale of a principal residence, no loss is allowed. If you bought the time share for investment purposes, you may be able to take the loss as a long-term capital loss.
The difference between amount realized and adjusted basis on the sale or exchange of capital assets. Long-term capital gains are taxed favorably. Capital losses are deducted first against capital gains, and then again up to $3,000 of other income.