I don't have a definite idea how long the work will take, previous plans have talked about the subway being closed for 3-6 months, which does seem like a long time for a relatively small job, but this would include some contingency for bad weather, especially if the work starts in September.

One word of warning, we have been given provisional dates for the works to take place previously, but this does seem like likely to go ahead after the Olympics than at any time in the last six years.