Not long, less than a couple of days I think. I wonder if the 651 visa is only available to UK or othe European nationals? Rules may be different for US citizens- but certainly worth trying!
I got the evisitor visa subclass 651 through the Australian gov website, link below. That one's free, visa 601 is not.
https://immi.homeaffairs.gov.au/visas/getting-a-visa/visa-listing/evisitor-651
Don't use a website that requires you to pay. It's free via Australian government website.
https://immi.homeaffairs.gov.au/visas/getting-a-visa/visa-finder