The authorities try to minimise disruption by carrying out work at off-peak times or during holiday periods. Often repairs are carried out fairly quickly, such as over a weekend, but they can be long-term: the extension of line C of the metro for example has had a knock-on effect on tram routes, and this will last months. On the positive side, the changes always seem to be carried out on schedule, and you can be confident that your journeys will not be affected any longer than necessary.