Correction : "ping -t" is under Windows and takes no parameter other than the IP address (continuous pinging).

Under Linux it's "ping -i" and you can (must) specify the interval between pings, in seconds. The following tests show that the "sleep-mode timeout" on the 770 is between 4 and 5 seconds :

$ ping -i 4 10.10.10.12
PING 10.10.10.12 (10.10.10.12) 56(84) bytes of data.
64 bytes from 10.10.10.12: icmp_seq=1 ttl=64 time=11.8 ms
64 bytes from 10.10.10.12: icmp_seq=2 ttl=64 time=3.11 ms
64 bytes from 10.10.10.12: icmp_seq=3 ttl=64 time=3.10 ms
64 bytes from 10.10.10.12: icmp_seq=4 ttl=64 time=3.61 ms
64 bytes from 10.10.10.12: icmp_seq=5 ttl=64 time=2.91 ms
64 bytes from 10.10.10.12: icmp_seq=6 ttl=64 time=2.81 ms
64 bytes from 10.10.10.12: icmp_seq=7 ttl=64 time=3.00 ms
64 bytes from 10.10.10.12: icmp_seq=8 ttl=64 time=2.95 ms
64 bytes from 10.10.10.12: icmp_seq=9 ttl=64 time=3.18 ms
64 bytes from 10.10.10.12: icmp_seq=10 ttl=64 time=3.55 ms
64 bytes from 10.10.10.12: icmp_seq=11 ttl=64 time=2.88 ms
64 bytes from 10.10.10.12: icmp_seq=12 ttl=64 time=2.97 ms
^C
--- 10.10.10.12 ping statistics ---
16 packets transmitted, 16 received, 0% packet loss, time 60005ms
rtt min/avg/max/mdev = 2.810/3.683/11.878/2.131 ms

$ ping -i 5 10.10.10.12
PING 10.10.10.12 (10.10.10.12) 56(84) bytes of data.
64 bytes from 10.10.10.12: icmp_seq=6 ttl=64 time=1892 ms
64 bytes from 10.10.10.12: icmp_seq=7 ttl=64 time=3.28 ms
64 bytes from 10.10.10.12: icmp_seq=8 ttl=64 time=2.88 ms
64 bytes from 10.10.10.12: icmp_seq=15 ttl=64 time=415 ms
64 bytes from 10.10.10.12: icmp_seq=22 ttl=64 time=1286 ms
64 bytes from 10.10.10.12: icmp_seq=23 ttl=64 time=2.98 ms
64 bytes from 10.10.10.12: icmp_seq=24 ttl=64 time=3.29 ms
64 bytes from 10.10.10.12: icmp_seq=31 ttl=64 time=2370 ms
64 bytes from 10.10.10.12: icmp_seq=32 ttl=64 time=3.53 ms
64 bytes from 10.10.10.12: icmp_seq=33 ttl=64 time=3.40 ms
64 bytes from 10.10.10.12: icmp_seq=37 ttl=64 time=2.99 ms
64 bytes from 10.10.10.12: icmp_seq=39 ttl=64 time=3.25 ms
^C
--- 10.10.10.12 ping statistics ---
39 packets transmitted, 12 received, 69% packet loss, time 189990ms
rtt min/avg/max/mdev = 2.889/499.257/2370.771/818.451 ms

Aggressive, indeed :-)