It is not expected that uv-coverage will be a significant issue as even snapshot observations will have sufficient uv-coverage for the vast majority of projects. You can verify this using the simdata task in CASA or the ALMA Observation Support Tool (http://almaost.jb.man.ac.uk/). The simdata documentation can be accessed directly at http://casaguides.nrao.edu/index.php?title=Simulating_Observations_in_CASA.
In the unlikely event that you do find you need more time than estimated by the ALMA OT in order to fulfil uv-coverage requirements, you should select "Override OT's sensitivity-based time estimate" in the Control & Performance editor of the OT, and provide a careful justification in the Technical Justification. This may be beneficial for observations of very complicated and bright sources, where the OT estimated time is too short to fully recover the structure. The time entered should include all calibrations and overheads.