Customs are a bit different by their nature – the script is largely front-loaded, meaning you don’t get as much of a separation of “stages” as the major titles. Working your way up to 5 minutes should be acceptable to get the entire script. Starting with 60s is fine, but 30s if you have recon.
EDIT: Note, the “cores” however, are split by the mini-stages. It’s the modules that we can’t really do like that.