Is it obligatory to continue donating 15% of one's income to the poor of Syria until the war ends, or only for the year in which the pledge was made, especially given personal debts due to a low salary?

If your covenant with Allah was an intention you did not utter, then nothing is binding upon you. However, if you uttered it, the preponderant view is that it becomes an oath and a vow if it is an act of obedience, and merely an oath if it is not an act of obedience. What is a vow must be fulfilled, and what is an oath gives you the choice between fulfilling it or offering expiation for breaking an oath. The duration for fulfilling the covenant is determined by your intention at the time of making the covenant. You are not obliged to fulfill a vow if you are poor or indebted and do not expect to be able to fulfill it; in this case, making a vow might even be forbidden. However, if the vow was made before you became poor or indebted, it is valid, and obligatory expenses and human rights take precedence over fulfilling the vow, and you expiate for the remainder by offering expiation for breaking an oath.