An approach for assembly time estimating following sequence and process modelling since the conceptual phase of new assembly systems design is proposed in this work. The purpose is to support the decision about their levels of automation (LoA) following a dedicated modelling language labeled ASML offering a better visibility about processes design and analyses. ASML provides an easy way of defining several systems alternatives based on an initial ASML model representing assembly motions using a standard vocabulary and structured executing architectures. The correlation of early phase modelling with time estimating proposed in this paper to support LoA decision is up to us new to the literature. The approach of modelling and time estimating was tested on assembly examples and a computerized tool was also developed and tested. Several related applications such as processes time performances evaluating, time savings, productivity assessment, lines balancing, and cost estimating are proposed as well as other possible future applications presented as perspectives and future works that can be supported by the defined environment dedicated to early phase assembly systems design and automation decision.