SRD said:A creature with the summon ability can summon specific other creatures of its kind much as though casting a summon monster spell
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Summoned creatures automatically return whence they came after 1 hour.
shilsen said:The description for spell-like abilities does say that it "has a casting time of 1 stanard action unless noted otherwise in the ability or spell description" (PHB pg.180). Since the spell description for summon monster spells does say that it takes 1 full round to cast, I'd say the same is true for it as a Sp ability.
This is pretty much what the Monster Manual (actually the SRD file Types, Subtypes and Abilities) has to say on the topic:The Monster Manual is the primary source for monster descriptions, templates, and supernatural, extraordinary, and spell-like abilities.
It does not rule out using the spell description, but also does not tell you to use it explicitly. The tactics section of various fiends is not helping either, since they do not note if the creature should adjust its position even when using Sp abilities based on spells taking a standard action to cast. I'd be inclined to use a standard action myself, but the thought to use the normal casting time has a certain appeal...Spell-Like: ... A spell-like ability usually has a limit on how often it can be used. A spell-like ability that can be used at will has no use limit. Using a spell-like ability is a standard action unless noted otherwise, and doing so while threatened provokes attacks of opportunity. It is possible to make a Concentration check to use a spell-like ability defensively and avoid provoking an attack of opportunity, just as when casting a spell. A spell-like ability can be disrupted just as a spell can be. Spell-like abilities cannot be used to counterspell, nor can they be counterspelled. ...