How do you determine what the HP of a summoned creature would be?

[Masakan]

In other words it's HD, Is it based on caster level or is it completely stagnant?

[ComaVision]

I don't know if I'd use the word "stagnant" but it is static, unless you have something affecting it (such as the Augment Summoning feat).

[Deadline]

In other words it's HD, Is it based on caster level or is it completely stagnant?

Usually, my group just uses the stats in the monster entry. In higher power games, we use max HP.

[marphod]

In other words it's HD, Is it based on caster level or is it completely stagnant?

It is as listed in the Monster Entry, which is (or should be) the average value for each Hit Die.

If you have Augmented Summoning, and the creature has a Con score, add 2 HP per HD (from the +4 to Con). Same for similar abilities.