As written, the kineticist utility wild talent kinetic cover has no listed duration. This is unaddressed by the Occult Adventures FAQ, mentioned as an issue once in the kineticist playtest thread from 2014 (and another user's followup post is of no help), and is unmentioned in class author Mark Seifter's ongoing Ask Me Anything thread. This last would likely be the most direct way to get a semi-official answer as to the duration of kinetic cover until and if Occult Adventures receives errata.
Without a listed duration, kinetic cover is created instantaneously, remaining in place until destroyed. As the created wall is both paper thin with no hardness and pitiful hp and crumbles under 5 lbs. of weight, most creatures can destroy it with ease. This would be fine except that "you can have a number of kinetic covers in existence equal to your Constitution modifier + 1/2 your kineticist level," and if, for example, a kineticist uses all his kinetic covers in a dungeon and teleports away, the kineticist must either hope that dungeon denizens hate walls made of water or whatever and destroy them or return himself or send agents to the dungeon to destroy his own kinetic covers just so he can make more. That's silly.
Were kinetic covers permanent yet dismissible, they become vulnerable to dispel magic effects, but then the kineticist can take a standard action to dismiss a kinetic cover. However, the rules for dismissible effects still require the effect's controller to be "within range of the spell's effect," and the range of kinetic cover is only 30 ft. While better than instantaneous, this isn't much better.
Rounds equal to the kineticist's Constitution modifier? Sure, I guess
The powers cloud, wall, deadly earth, mobile blast, and others have this duration. It's long enough to matter in combat but not long enough to cripple the power were the kineticist instantaneously transported a vast distance from his kinetic covers. Extending the duration to longer rounds runs the risk of the previous covers' existence preventing the current use of any covers, and that would suck. In fact, were I playing a kineticist, I'd argue against the GM trying to impose a more typical duration like 1 min./level or 1 hour/level. A low-level kineticist needs to be able to use his powers, and there's a point when the kinetic cover talent can be a kineticists's only utility power.