It hasn't been published yet in the dragon magazine but it would be deliriously high.
That's because they can use their psionic abilities at will. For a PC that's an incredibly useful advantage since they are always "on the scene". They can use these abilities every fight, several time per encounters.
It's almost impossible to balance correctly ECL for creature that have these kind of abilities. The Nymph and the Mind flayer (ECL 12 and 18) have the same problem. If you raise their ECL to a point where their abilities are useful but not disruptive, they have so little HP that they can't survive most fight. If you you put their ECL at a point where they can have enough HP to hope for survival, their special abilities wreak havoc with the campaign.
The psionic template, like the mind flayer and the nymph, are simply not very appropriate for a PC without some modification.
What you should do is play a standard monster and give it psion level. Here you go; that's a psionic [insert name of creature].