A small Drupal site using a pre built distribution or Drupal CMS can be up and running in one to two weeks. That assumes straightforward content, a stock or lightly customised theme, and no complex integrations.
A typical mid sized custom build takes eight to sixteen weeks from kickoff to launch. That covers discovery, design, content modelling, theme development, content migration, testing, and training.
Larger enterprise projects often run six to twelve months, particularly when they involve legacy migrations, multiple stakeholders, or integrations with CRM and marketing automation tools. The biggest time sinks are usually content preparation and stakeholder review cycles rather than the technical build itself.