I cannot think of one good reason why men should have to wait any length of time to determine paternity - unless the mother has devious motives. Such an extremely personal, and possibly painful, issue should be handled in an adult-like fashion. In my opinion, men get the short end of the stick when comes to paternal rights.
Circumstances where paternity tests are needed or requested are sensitive matters. In this day and age of sexual promiscuity and broken homes, children are often the ones most harmed by legal struggles that ensue. However, this should not mean that a father's right to know if a child is in fact their own. Mothers placing children up for adoption may not notify the father of the child's existence; this situation should not preclude a parent from determining more than two years later that a father has rights. Furthermore, in cases where child support monies are in question, fathers should have the right to determine that they are in fact paying out for children they are in fact related to.
A person should be allowed to become a father whenever they want to. Even a child who is much older wants to know where they came from and develop a relationship with their father. Sometimes, mothers hide their children from the fathers on purpose. This isn't fair to the father or to the child.
No, there should not be a time limit on paternity questions. Paternity questions can rise at any time during a child's life. There is no time limit on deception or doubts and if a woman is not given a time limit to declare a person's paternity, then a man should not have a time limit, either. This type of question should be answered any time it arises.