When the federal government introduces a bill, the Basic Law gives the Bundesrat six weeks to comment. If the Bundesrat asks for more time for an important reason, “insbesondere mit Rücksicht auf den Umfang einer Vorlage”, especially in view of the length of a bill, the period becomes nine weeks.
The government may mark a bill as especially urgent. It can then pass the bill to the Bundestag after three weeks, or after six if the Bundesrat asked for more time. For amendments to the Basic Law the period is always nine weeks, and the urgency rule does not apply.