It depends on the scale of your project. For example, if you aim to build a CRM website with typical functionality and minimum third-party software integrations with a simple interface design, it can take approximately 6–8 months to be built.
In case you need to create your own CRM system for iOS and Android, it will take you extra 2 months. It shouldn't go unspoken that if you need to transfer a big amount of data from your legacy CRM system to the new one, the migration process can seriously impact the terms of your project development.