Via: http://minecraft.gamepedia.com/Lapis_Lazuli_Ore#Natural_generationhttps://minecraft.wiki/w/Lapis_Lazuli_Ore#Natural_generation
Lapis lazuli ore is found at a depth of 31 and below. The highest concentration of lapis lazuli ore is found between levels 13 and 16. At this level the concentration is about 0.083% of all blocks (0.1013% of stone). The concentration drops linearly as one gets farther from these rows. Overall, lapis lazuli is around 1.1 times as common as diamond ore with an average of 3.43 lapis lazuli per chunk.
If you are chunk mining, I would recommend moving to another chunk.