Jamieson, Fausset and Brown on Job 2:13

13. seven days ... nights--They did not remain in the same posture and without food, &c., all this time, but for most of this period daily and nightly. Sitting on the earth marked mourning (La 2:10). Seven days was the usual length of it (Ge 50:10; 1Sa 31:13). This silence may have been due to a rising suspicion of evil in Job; but chiefly because it is only ordinary griefs that find vent in language; extraordinary griefs are too great for utterance.