i have make some test with millis() between i receive < and > and when i send for exemple "<255>", he give 5ms «, i think 20ms is good for a timeout...
Once again, I thought the protocol was <, letter, value, >...
9600 baud is approximately 1 character per millisecond. With a packet that can be 7 characters long, the largest packet should arrive in 7 milliseconds, so 20 milliseconds is probably not a bad time limit.
You can record when the < arrives, using millis.You can record when the > arrives, again using millis. If the difference in times if greater than 20, ask that the value be sent again.
However, I don't see the benefit of this. The long delay you have on the sender will make it wait 1 second before it sees the request to resend the packet.
Something more like a standard RC transmitter, where the sender sends data at close intervals, even of the data is the same as what was sent last time, might be a better idea.