Is this being done through Management Studio, if so, are you able to script out the change and see what is occurring, is the change creating drop and recreation and reinsertion of data into the table, are any other tables affected, FK constraints etc. Is the change going to take some time, if so, if it is not production, you are able to take potential downtime while it finishes, script out the change and run the script.
Management studio does a lot in the background after you make changes in design mode depending on your schema?
Andrew