Many people in the Bible fasted in a variety of ways. Daniel fasted for three weeks (Daniel 10:2-3). Jesus fasted for forty days (Matthew 4:2). Moses fasted for 40 days (Exodus 34:28). There are no specific commands concerning how long a personal fast should be.

In Daniel 1, Daniel and his friends fasted from certain foods only. In Daniel 6:18, there is mentioned a single night of fasting: "Then the king went off to his palace and spent the night fasting, and no entertainment was brought before him; and his sleep fled from him"

Saul fasted for three days, Acts 9:8-9 "Saul got up from the ground, and though his eyes were open, he could see nothing; and leading him by the hand, they brought him into Damascus. And he was three days without sight, and neither ate nor drank."

In Acts 13:2-3 it appears that they fasted until the Holy Spirit spoke to them: "While they were ministering to the Lord and fasting, the Holy Spirit said, "Set apart for Me Barnabas and Saul for the work to which I have called them. Then, when they had fasted and prayed and laid their hands on them, they sent them away."

So there is no set length or rules for fasting, and it is not for the purpose of making us holy so that God will hear our prayers. God hears our prayers because we pray in the name of Jesus, being saved and made holy by faith in Him alone. So, the answer to your question is Yes, God hears your prayers always. Maybe you are called to fasting and prayer for shorter periods of time; like one or two days a week. It's presumptuous to make up your mind to fast until God answers; a fast is not intended to pressure God into doing what we want.

Fasting is a ritual that simply heightens remembrance.  There is nothing particularly righteous about the act of not eating; it is the intention of the heart that drives you to not eat or to deny certain foods and the subsequent prayer that is pleasing to God.  For instance, a chocolate lover would fast from chocolate for a time in order to heighten their remembrance of some issue they are devoting to prayer.  Fasting can range from total denial of all food for a short time to very specific abstinence from things that would represent a sacrifice of some sort.  The focus is always on a sacrifice of food that reminds us to pray.