You cause the constrict damage once per grapple check.
Since each attack from a creature with the grab special ability allows a different grapple check, for each grapple check that succeeds, the creature also causes an additional constrict damage to their target, including on the first grapple check.
As releasing a grappled target is a free action:
Although both creatures have the grappled condition, you can, as the creature that initiated the grapple, release the grapple as a free action.
The number of times you can do this in a round is only limited by your GM:
Free actions consume a very small amount of time and effort. You can perform one or more free actions while taking another action normally. However, there are reasonable limits on what you can really do for free, as decided by the GM.
However, if the creature decides to not release the grapple and continue attacking with the other tentacles, it suffers an additional penalty:
The creature has the option to conduct the grapple normally, or simply use the part of its body it used in the grab to hold the opponent. If it chooses to do the latter, it takes a –20 penalty on its CMB check to make and maintain the grapple, but does not gain the grappled condition itself.
So, after the first succesful grapple check, the creature has three options:
Release the grapple and continue her full-attack action;
Keep the grapple, gain the grappled condition herself, and end her full-attack action;
Keep the target held with her tentacle and continue her full-attack action, but this grapple check has a -20 penalty.
What the developers have said about this?
When discussing the mechanics of grab and releasing the grapple as free action on a non-official thread, James Jacobs (Paizo's Creative Director) has said this:
The grab ability does NOT allow a monster to maintain a grapple for free; that action is still a standard action. In most cases, it's better for a grab monster to simply release its grabbed foe at the start of its turn and attack again to trigger its grab attacks. It's a weird side effect of how the rules work.
Also, as additional notes, that constrict damage happens as soon as you make the CMB check, and that Greater Grapple mixed with constrict is mean.