What do you mean by twelve machines? Twelve servers? Twelve workstations and a server? Without knowing what portions of SBS they are actually using, how big the actual total amount of data is, available methods of data transfer, and so on it would be hard to say how long it might take. I would allow for much more than a weekend to do it properly and safely. However the good news is that Microsoft allows for multiple SBS servers to be up and running on the same network for a period of time (30 days I think) to perform a smooth migration and change over. Microsoft does have a full explaination on the migration process HERE
My best advice is to read that entire actricle and it's sub-topics totally. Formulate a WRITTEN plan on paper so you do everything in the proper order and do not forget the next sometimes crucial step. It is critical all of the steps are performed in the correct order for a successful migration.
Get your facts first, then you can distort them as you please.