Depends on the tire compound/structure, the weight of the bike+rider, and its intended usage.
Manufacturer recommendations take into account weight and OEM tire compound/structure. (This is why the recommended pressure for my Harley changed significantly in the next model year when the OEM tire manufacturer changed).
What you're basically looking for is a specific level of grip (either maximum grip, or for a balance of grip and low wear). A heavier bike and/or rider means more tire deformation, which means it heats up more, which means you need to increase pressure to compensate. The opposite is true for a lighter bike/rider.
A softer tire compound means that a higher pressure is needed to hit the same ratio of grip and tread life as you would get with a tire having a harder compound. Conversely, a harder tire should be run at a lower pressure so that it heats up sufficiently.
OEM-wise, I've seen recommendations from 30 PSIG cold through 42 PSIG cold. The 42 is on my ZX-14R, which is exactly what I run at. This is high for a sportbike probably because the tire is a softer compound, and especially because the bike is over 100 lbs. heaver than most other sportbikes (as are its typical riders
).
Cruisers tend to be rear-heavy and more likely to carry passengers, so 36-41 PSIG cold in the rear is common. This is lower than the ZX, despite being heavier, probably because they also tend to run harder rubber, which needs to heat up more for proper grip.
As mentioned by someone else, for sportbikes and racing you'll typically see pressures around 30 PSIG versus 35+ PSIG for regular street use--these are good starting points. If you know the weight at each tire's contact patch, you can calculate from there what the front should be versus the rear. Adding a passenger to the bike means that you should add air to the rear, often as much as 5 PSIG.
For street use, a good rule of thumb is to set the cold pressure to whatever level results in a 4 PSI increase when hot. Or, in other words, a 45 degree fahrenheit temperature increase. You should be able to hit this mark within 10 miles of freeway riding.
Also remember that OEM front recommendation also tends to take into account the rim hardness and therefore susceptibility to pothole deformation. A higher PSI here can protect your rim.
"Cold" inflation pressure means the pressure in the morning (at the lowest temperature for the day) before the sun hits the tire and heats it up.
Trivia: 29.4 PSIG is literally three tires' worth of air (at standard atmospheric) stuffed into a single tire. 44.1 PSIG is four tires' worth of air. See: Ideal Gas Law.
Tire pressures
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