Whenever you cast a conjuration (summoning) spell, increase the duration by a number of rounds equal to 1/2 your wizard level (minimum 1). This increase is not doubled by Extend Spell. At 20th level, you can change the duration of all summon monster spells to permanent. You can have no more than one summon monster spell made permanent in this way at one time. If you designate another summon monster spell as permanent, the previous spell immediately ends.