You can legally download the OVA and run it free for 90 days with sufficient throughput for most labs and certification exams. After 90 days, the router will continue to pass traffic at only 2.5 Kbps (effectively halted). You can re-install or convert to a paid license.
To successfully download the CSR 1000v OVA from Cisco, ensure you have: cisco csr 1000v ova download free