LMS development process consists of several stages: writing a project brief, idea validation, design and prototyping, development, deploying to production, and post-launch support. Each of these steps results in certain deliverables used in the next step.
Thus, the total time needed to build your LMS will vary depending on how clear and feasible your requirements are, how well-researched your target market is, etc. Then there is the learning management system design complexity, whether or not you need a prototype, how heavily your LMS relies on third-party data, what platforms you’re developing for, and some more.
To throw in some numbers from our experience, developing a custom LMS from scratch can take around 4–6 months or more.