Speak with Dead takes ten minutes to cast, but this ability lets you use it for rounds per day equal to Oracle level.
Since it is unlikely that the Oracle PC will reach level 100, we can assume that the casting time is not included. But that brings up further questions:
Is it simply a standard action to use the ability?
Is it a free action, but only lasts for nonconsecutive rounds equal to level, starting with the round you activate it? If so, what if the spook makes its will save? Does the round you first attempted to use it count?
Do you have to take the ten minutes to cast (use?) this ability "as the spell", but the duration is simply swaps rounds/level for minutes/level?
If the spook is evasive, does this count towards your rounds/day? Seems like a rough GM could really nerf this ability.
You aren't casting the spell Speak With Dead, you are using a standard action (as is the default when activation action is not mentioned) to activate a supernatural ability of your character which works just like Speak With Dead. It specifically mimics the spell's description, not its parameters like casting time or duration or range.
What do you mean by evasive? You mean it hems and haws and rambles and wanders off topic for a minute or two? Well, yes, that is the GM's choice in keeping with the target's personality. Might work out better if you ask simple yes/no/one word answerable questions.
Talk to your GM about how this would work out in practice. If he'd blow the entire day's allotment of rounds just to answer a single question, it isn't a very useful ability. If he agrees that a single round gets you a simple question and single word answer, it may be better.
Remember, a round is only 6 seconds, so your question would have to be very quick and simple. Also, once you activate it, you either have to ask all the questions you have in that block. You cannot use Voice of the Grave on the same target more than once a week (a limitation of Speak With Dead to which you must also adhere)