Every such project usually takes twice as long as anticipated. Medium size jobs take about six months while major refurbishments run close to a year, with some exceptions that I'll explain. As with everything in life, the better the planning, the smoother things go. It is extremely important that even medium size jobs be well-planned. Projects can turn into disasters due to a lack of planning.
Every hour spent on planning can save dozens of hours of wasted effort.
Before even considering the project, first seek out the most experienced project manager you can find. Get professional help and pay him to size the job up for you and give you a rough estimate of overall cost. This should be done before you complete the purchase. Once you're comfortable with the rough numbers, then you can complete the sale and move on to detailed plans and estimates. Anyone who has done several major refits can fairly easily rough out the overall cost.