how deep should it be for these to be effective?

I'm assuming 15 plus so you can read it on your graph?

Not an expert with them but I have caught bass 8 to 10 foot directly under the boat in murkier waters .

I cast them to shallower water (8-10 ft or less) all the time, working them over flats and down drops, but also vertical jig them in anywhere from 12’ to 35 ft of water.

The jigging spoon is one of my favorite baits.  I don't normally use them in less than 25' of water and will fish them as deep as needed though rarely deeper than 100'.

They will work at any depth, but they shine in deeper water. You can fish them anywhere between horizontal and vertical. I usually don't fish them in water shallower than 10-12 ft, but they will work. 

For jigging, about 10'. I use them for schooling fish when you only need a couple feet though. 

Jigging spoons work year around when bass are feeding on schooled bait fish. Select a spoon with slower rate of fall (ROF) for shallower bait schools.

Tom

You can bomb them and bounce off the bottom at any depth, don’t need to see it on graphs