First I wanted to congratulate the designer who created the mech with 3DS Max, really wonderful. I would like to know how much time is needed to complete one
Welcome. It totally depends on the Artist, his/her skill level, and how much free time they have. Mechs have taken anywhere from 2 weeks to 2 months. Personally, If I had all the free time in the world I could pump out a Mech every week or 2. I don't have all the free time in the world tho =(