How long to transfer 300 K to new mac?

Nobody's mentioned Gigabytes, GBs. That more likely how much data you need to transfer. The time will be dependent on how you're transferring the data, i.e. what app is being used to to the transfer and how the two Macs are connected. In any case don't connect them via WiFi. Connect them by cable, preferably Ethernet.

So give us more info on the size of data and how it's to be transferred and we can estimate a time, emphasis on estimate.

K maybe a second. Meg or terabyte a heck longer.

Do you mean 300 kb or 300 mb ? If it's 300 kb, I agree, hardly any time at all. If it's 300 mb it'd be around 30 min.