VPN IP Lease time

It’s not DHCP for the IPsec’s mode config IP assignment. So there is no concept of “lease time”. And once a client disconnects, the IP would be returned to the pool for other new users. However, if you’re using RADIUS server for the user authentication, you should be able to use “Framed-IP-Address” attribute to specify a particular IP for each user. But to do that, you need to have enough IPs for all possible users and pre-assign them to all at the server.

Toshi

FortiGate does not provide a configurable lease time specifically for SSL VPN IP ranges — SSL VPN IP pools are managed directly by the FortiGate process, not by a DHCP lease mechanism. When a user disconnects, the assigned IP is immediately released back to the pool and may be reassigned to another user.

You can Assign fixed IPs by user or group:
Map a unique IP or IP pool to specific users or user groups in SSL VPN portals.
Use RADIUS attributes (e.g., via FortiAuthenticator) to push a static Framed-IP-Address per user