Having worked in many different types and sizes of companies, I have always found there to be a great variance in how long it takes to accomplish certain tasks. For example, one organization might have lots of hardware at it's disposal, while another might need to order hardware. Or, another good one is the task of setting up DNS records. I've been in places where I can simply log into Active Directory myself and create what I need without hesitation, while other places can take upwards to a week to get a DNS record pointing to an IP address.
Whatever the case, the majority of the time it is the company's processes and resources that define how long something takes.
This week, I've been using the Program Evaluation and Review Technique (PERT) to estimate the duration of an extranet portal project. Fortunately, I have a very good sense for the company's processes so I can factor these things in fairly accurately. Still, I really like to include PERT calcualtions in my project plan because it allows me to factor in optimism and pessimism and come up with an accurate duration.
With PERT you calculate a weighted average by doing this:
Optimistic Time + (4 * Most Likely Time) + Pessimistic Time / 6 (Schwalbe, 2009)
So, if I wanted to estimate the time it takes to spin up some virtual machines and configure a SharePoint 2010 Farm completely from scratch, I might do something like this:
3 workdays + (4 * 5 workdays) + 10 wordays / 6
Result = 7.5 workdays
Reference
Schwalbe, K. (2009). Information Technology Project Management (6th Edition). Boston: Course Technology.
