With IPHost Network Monitor you can run simple snmp requests against a Fortinet device in your network.
fgVpnTunEntLifeBytes
fg Vpn Tunnel Ent Life Bytes
1.3.6.1.4.1.12356.101.12.2.2.1.16
Lifetime of tunnel in bytes, if byte transfer based lifetime used
Back to FORTINET-FORTIGATE-MIB MIB page.
IPHost Network monitor allows you to monitor fgVpnTunEntLifeBytes on Fortinet device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Fortinet WAN extenders right now.