$$t = \fracVQ_s \times \ln\left(\fracP_aP_a – P_v\right)$$
Have a specific application? Drop the specs (part size, material, cycle time) in the comments, and we will run the numbers for you. vacuum ejector calculation
$$Q = 13.2 \times d^2 \times (P + 14.7)$$ cycle time) in the comments
But here is the common pain point: An undersized ejector leads to dropped parts; an oversized one wastes massive amounts of compressed air. vacuum ejector calculation
$$Q_s = Q \times C_v$$