There are two sets of changes. Some are initiated by the Federal Constitutional Court when it examines the electoral law, or in the states by the state constitutional courts. This is then amended promptly, more or less, as demanded by the court.
Other changes, which are more of a reform, are decided on the political level, and they take a while. I cannot cite any time span, but the reforms in the states of Hamburg and Bremen to leave the closed list and to establish a preference vote took quite a while because they were initiated from the outside and carried into the parliament. This may have been longer than four or five years, or two legislative periods. I don't really know. I would have to look it up.