regarding your question:
I suppose you are using the UDP ASCII Interface.
In general all devices are sending ACKs. The only difference is, that the latest FW versions do send the ACK by default to the UDP source port. Older versions are sending the ACK to port 21000. If you want to have the ACK sent to the source port simply use the option "ps" when sending: "snd sony_tv,vol+,ps".
Of course the newer devices will accept this option, too.
In general you will need to wait until the ACK to send a new command. Otherwise commands might get lost.
You can send the next command immediaetly after the ACK.
It is difficult to wait a fixed time because the time depends on the length of the IR Code itself.