What is the most effective frequency for 1-on-1 meetings with direct reports?

If direct report then every week is needed. If more than direct report then bi-weekly or monthly depends on what is your current focus.

I have 1:1's weekly with each of my direct reports who range from a project analyst to mostly Directors and Senior Directors. Given level of work and strategic initiatives we are working at the moment, weekly and even more frequently is necessary to stay in lock step. Less frequent is fine when things are slower. I meet with my CFO weekly unless travel and other priorities take place in which case, we email or text as needed for anything as FYI or urgent matters. The aforementioned approach has worked very well for me for the last several years, especially having been in a remote environment since COVID. 

Use 1:1's for individual develeopment and coaching.   Not as a place where the work happens.   

Put as much of the work out in the open via Teams or slack channels so that information and connections aren't lost in 1:1's.   use the 1:1 for the often neglected human things, like processing, life stuff, vulnerable conversations, coaching.   

Weekly 1:1s plus as many live, ‘on demand’ deal coaching calls as are needed to win, based the cadence of a deal.

1 -1 on weekly basis really keeps the employees engaged and motivated. Alternate weeks just discuss the employee IDP and growth and no project status.