Hi Neil,
You break very large jobs down to manageable sections and don't think of the totality of it, especially if you're not used to such big jobs.
Each section then becomes a job to which you allocate a block of time so that the job is now about time. You then work out how many individual blocks of time equal the totality of the whole job. This gives you a time frame that you can present to the client.
You can reduce the overall time by getting in extra help so that more jobs, or as they are now, blocks of time get done in a day and so reduce the time taken to complete the whole job.
Simon