While it's not called out, it appears to be an at will ability. So, no limit.
LordKailas is correct. This is not a spell-like ability, but just something you can do; there is no need to activate Feral Speech and no duration limit.
I think you do need to activate it. Since it’s a supernatural ability, it should take a standard action. And it should have the minutes per level duration of the spell.
Feral Speech (Su) (Ultimate Magic pg. 86): You gain the ability to speak with and understand the response of any animal as if using speak with animals, though each time you speak to animals, you must decide to communicate with either amphibians, birds, fish, mammals, or reptiles, and can only speak to and understand animals of that type. You can make yourself understood as far as your voice carries. This discovery does not predispose any animal addressed toward you in any way. When you reach 12th level, you can also use this ability to communicate with vermin. You must be at least a 5th-level wizard to select this discovery.
Supernatural default to a standard action if no other action is specified. Speaking is explicitly a free action. The effect applies any time you speak (which is a free action) and requires no independent activation. If it did work the way you suggest it would be listed as a spell-like ability rather than supernatural.
The effect applies any time you speak (which is a free action) and requires no independent activation.
I don't believe the text says or suggests that.
If it did work the way you suggest it would be listed as a spell-like ability rather than supernatural.
That's just false. Supernatural abilities can work like spells too. The witch has multiple examples of these. And has it's own version of Feral Speech:
Feral Speech (Su) (Ultimate Magic pg. 81): This hex grants the witch the ability to speak with and understand the response of any animal as if using speak with animals, though each time she uses the hex, she must decide to communicate with either amphibians, birds, fish, mammals, or reptiles, and can only speak to and understand animals of that type. The witch can make herself understood as far as her voice carries. This hex does not predispose any animal so addressed toward the witch in any way. At 12th level, the witch can use this hex to communicate with vermin.
Unless otherwise noted, using a hex is a standard action that does not provoke an attack of opportunity.