There are various strategies that can be used to keep the index up to date.
SearchUnit provides the option of installing the Windows Service, and configuring it to reimport sources on any given index directory at specified intervals. More information on the administration installer and Windows Service
Through the Windows Index Management tool's Publish form, it is possible to setup a scheduled task to reimport sources and deploy the result via xcopy or ftp. This approach is ideal when a service cannot be installed, and it facilitates updating the index from a development or staging machine. More information on 'publishing'
If there is application code running when changes are made to content that should be indexed (eg. an ASPX page runs when a document is uploaded, or deleted), then it is possible to immediately update the index using the SearchUnit API. This approach is ideal when it's possible to implement, as it results in a fresher index (no waiting for periodic updates) and is more efficient than the other approaches (since it does not require scanning files/folders). More information on incremental updates