We are a product of choices and decisions, and a woman will one day decide whom to marry. There is no harm in a man proposing to a woman, but it is up to her to decide whether to accept or not.
Before making a decision about whom to marry, be convinced of your decision because you will live with that decision for a lifetime. Ultimately, you are responsible for your choices and decisions. When your decision is backed up with strong convictions you are not likely to regret it in future. Having strong convictions is a combination of some elements which include whether you have prayerfully considered the proposal; whether you have carefully thought about it and the destination it will take you in future, because when you choose to marry someone and not another person you are headed in a different direction. In addition to these, you need to consider if he is the right man for you, and whether your destiny and long-term vision are in synchrony. Having these factors combined together will help you have a strong conviction upon which you can base your decision.
You have the right to say no because you have a free will; and if your decision is no, you should not feel bad about it. However, when you give no to a proposal, let it be in an honourable way; you should not abuse, insult or be rude to the person. You could even give reasons for your rejection of the proposal. For example, based on your personal convictions, your purpose and vision in life do not synchronize with the man’s own, you may wish to say no.
Don’t allow your response to delay or drag for too long, either it is a yes or a no. If it’s taking 6 months or over a year to make a decision, I think that is rather long. Be decisive and stand by your decision. However, this is not to say that you should be too hasty in making a decision or giving your answer. Take time to pray and think about it; give it some thought and consideration and come up with a solid decision. I believe this should not be more than three months after the proposal, though there could be peculiar cases and circumstances.
Face to face
As much as possible, give your reply in person and physically. An adage says ojulorowa i.e. it is more effective when you talk to someone while making eye contact. There are some discussions that are not suitable for the phone; something as important as responding to a marriage proposal should be discussed when you meet or see each other physically. Except in a case where one of you is located at a distant place, and there is no chance of meeting anytime soon, then you could communicate electronically or through the internet.
Marriage decision is a life time decision, get God involved in it; He will guide you on the right path.