Is it wrong for girls and boys in the church to be friends? How should we act toward each other and how much time should we spend together?
If anyone should be friends it should be members of the church. This is true for adults, and should be especially true for their children. Especially in the teen years, but even before that, it is important to have friends that understand and agree with your beliefs. When peer pressure is so great, for children or adults, it is good to have someone you can talk to that can help you through temptations. This is probably at least part of what Paul had in mind when he wrote, "Brethren, if a man be overtaken in a fault, ye which are spiritual, restore such an one in the spirit of meekness; considering thyself, lest thou also be tempted. Bear ye one another's burdens, and so fulfill the law of Christ." (Galatians 6:1-2)
Besides that, if boys and girls in the church can't be friends, how could a young man or woman find a Christian mate? I have seen many marriages where one spouse did not share the faith of the other, and many have had problems. I have seen people brought to Christ by a mate, but I have seen as many or more who have left their faith in such a circumstance. I am certainly glad that I married a Christian woman.
How much time should boys and girls in the church spend together? As much time as possible. Churches with large, active youth groups grow children who are likely to remain faithful when they eventually leave home. Maybe if our children spend a lot of time together then as adults they will keep up that habit, rather than having churches where the adults only see each other at the church building one to three times a week. How much time should a boy and girl spend together, not as part of a group? As much time as possible, but also as supervised as necessary. "Best friends" and "boyfriends/girlfriends" may spend more time together than others (and it is possible for a boy and girl to be only best friends), but that is to be expected.
How should you act toward each other? Since you don't specify an age group I am not sure exactly how to answer. Very young children generally act between boys and girls much like they act with others of the same gender. For some unknown reason I suspect that you are talking about teens. That is a little more complex, because teens are generally controlled more by hormones. Teenage boys and girls are more interested in each other for reasons other than mere friendship, and that makes relationships more complicated. One would hope that Christian boys and girls have been taught what types of behavior are inappropriate before marriage, but that is not necessarily the case. Teens should act with each other in a way that they are not overly tempted to sin. Beyond that, they should act with the same respect for each other that they owe anyone else.