There are rules of thumb for the required thread engagement but most of them are for coarse threads in hardened steel nuts so they do not apply well here.
The correct way to do this would be to figure out what the thread stress area is (it is a function of the diameter and pitch as well as number of threads and can be easily looked up in Machinery's Handbook). Then, you can look up the mechanical properties for the seacock and nut material (note: both need to be strong enough). Multiplying the stress area by the yield strength of the material will give you an axial force which can be transmitted. When you do this calculation for a fastener, it is straight forward since you just need to make the threads stronger than the rated yield strength of the fastener. For a thru hull, the same is true except that knowing the strength of the thru hull is a bit tougher (although a force/area approximation is a good start). Additionally, it is always wise to check the induced tension from tightening the nut and make sure it is below the thread yield strength and overall fitting strength. There are many online calculators that will let you figure this out, you just need the diameter and tightening torque.
Since this is a marine environment, you are going to want to leave yourself a significant factor of safety. There really is no excuse not to, it won't add a lot of weight. The marine environment has the unique ability to corrode or otherwise degrade just about anything which will greatly impact the strength. In addition, the normal fatigue assumptions about a bolted joint do not apply very well here because the fastener (thru hull) is stiffer than what it is clamping together.
Sorry that I didn't give you a number answer, there are too many variable. It will give you a place to start (or maybe just convince you to buy the longer ones).