The timescale for the migration varies depending on a number of factors. Much of the time is dependent on the number of data loads needed and the extent of the data checking that is needed.
If the original data is particularly poor in quality or structure or both, it may take considerable time to reach a point where it is deemed suitable for live operation. The time taken to reach this stage can also depend on library staff availability, since the burden falls on them to check data and feedback to the project team.
No one else is in a position to determine how data should be structured and if it is ready. Some projects take only one data load, if the data is straightforward. Others can take 4 or 5 months especially where data is not provided at the outset but appears late in the project or where the data provided is very poorly structured and needs multiple loads.