there is probably some sort of maximum initial spin rate, and even given that rate the planet might be guaranteed to be tidally locked at this point.
Glad you answered your own question. We have a good idea of what rotation rates are possible when planets form in a disk, probable rotation rates are basically a function of composition and mass (very small objects such as small moons, asteroids, and fragments are more complicated because their rotation rates are going to be affected by frequent impacts, but even then there's a limit to what gravity can hold together)
Basically, the planet in question--Gilese 581g, is very very very old. It orbits a red dwarf star whose lifetime is in the billions of decades--20-30 billion years likely (too lazy to check for an actual figure, but it's much longer than the 10 billion years for our sun). Based on the current age of the system it (and apparently every other planet in that system, from the bottom of the wiki page on tidal locking) should already be locked.
The estimated age of the universe is 13.75 ± 0.17 billion years. Where do you guys get off voting this drivel to +5 informative