1ug/ml is the recommended concentration for DAPI staining.
References:
1. A simple and robust protocol for immunostaining Arabidopsis pollen nuclei. SpringerLink. https://link.springer.com/article/10.1007/s00497-018-00360-7
2. Protocol: Staining Cells with Hoechst or DAPI Nuclear Stains. Biotium. https://biotium.com/tech-tips/protocol-staining-cells-with-hoechst-or-dapi-nuclear-stains/