How do you calculate the depth of penetration in a Schlumberger array?
Short answer: 0.2 times the total length of your cable(s).

100m rollout --> ~20m depth

Bear in mind that the deeper you get, the more you have to be careful with your data interpretation. Also if you do a roll-along your depth doesn't get deeper. 

Long answer see Bernard J (2003): Short note on the depth of investigation of electrical methods. Heritage Geophysics.