Nelson,
It's notoriously tricky to accurately schedule concept design for a major project, but a here are a few tips:
1) Make sure you know how "completion" is defined. Clients often give a vague brief for concept design, and it's important to nail down precisely what is required eg:
-Should a cost estimate be provided? If so, to what target level of accuracy? (eg +/-50% you can do as a desktop excercise, +/-20% and you'll need to be engaging with vendors) Just CAPEX, or should OPEX estimates also be provided? (NB: reent projects I've worked on in the UK have insisisted on CAPEX, OPEX and CARBON cost estimates!)
-Similarly, should a duration estimate be provided?
-Does the cleint have to approve the concept design? If so, against what criteria?
-What design deliverables are to be included? eg plot plans? PFDs? Operating Philosophy? Risk Register?
-Is any work associated with tendering the design/build contract included? eg writing the scope of works? Response to queries? tender assesment?
-What surveys are required? are desktop surveys sufficient, or will site visits be needed?
-What external stakeholders are involved, and who is responsible for dealing with them? (eg planning permission, landowners, environmental impact, utilities, liscencing authorities, etc)
2) A balance has to be struck between ensuring enough time is spect considering different concepts to ensure the best one is chosen, and minimising the wasteds work designing concepts that are not ultimately chosen. Typcally a Stage-Gate process is used to do this. eg:
-Stage 1: a couple of weeks are spent investigating all possible concepts and ranking them in terms of cost and technical suitability. The stage ends with a deicsion to select the best 3 or 4 concepts to further develop (this could be via internal meeting or with client).
-Stage 2: a few weeks are spent developing these concepts further to look for major stumbling blocks, and to refine the costs a bit further. The stage ends with a deicison to select the final concept to progress with (this could be via internal meeting or with client)
-Stage 3: Final concept is developed to required level of detail to complete the concept design contract.
3) If you follow a stage-gate process like above, accept that this means you can't have a detailed schedule until the latter stage, becuase you won't know what you are designing until then. You have to constantly review and revise the schedule as the scope of works gradually becomes clearer, and everyone needs to understand this can mean significant movement in the forecast completion date. -It's a version of 'rolling wave planning' really where you try and get some clarity about the work in the near future (say for the work leading up to the next key decision point), and accept that anything other than some high-level sumary bars and key milestones for the later works is unrealistic.