hi Erik,
you should see the timeout in the output of "show radius". The default of 5 seconds should hold good for most deployments. Increasing the timeout means more memory on BNG would be used to store the data (in case re-transmission is required).
RP/0/RSP0/CPU0:our9001#sh radius
<snip>
Server: 172.31.1.1/1645/1646 is UP
Address family: IPv4
Total Deadtime: 0s Last Deadtime: 0s
Timeout: 5 sec, Retransmit limit: 3
Quarantined: No
<snip>
hope this helps,
/Aleksandar