http://www.cvm.uiuc.edu/petcolumns/showarticle.cfm?id=77

I hope that the problem just started today, because as you know (and Im
typing this out for those people who don't know) urinary blockages are
DEADLY for cats.  You don't have much time once they are blocked, and a trip
to the emergency vet really is in order.

If he is just acting uncomfortable and has no other symptoms at this point,
you are PROBABLY safe.. (please rely on your own judgment of how the cat is
acting) to wait till morning.  See if you can feel his bladder.  If it is
the size of a baseball or small orange, you might want to reconsider
waiting.

I've had two different cats block three different times, it is never easy.

For future reference, feeding a high quality canned food should help
eliminate the problem.  Keeping a cat well hydrated keeps the bladder
flushed out.  dry food keeps a cat in a constant state of dehydration.  Cats
usually don't drink enough water to compensate.

Again, (as I love to refer to this website) www.catinfo.org explains diet
really well.