Once a day. More often if I’m turning cast iron and wiping the swarf/dust/oil off with a rag.

I just do it whenever it starts to feel “dry”. It’s most obvious when sliding the tailstock back and forth. I apply oil lightly and wipe off the excess.

It might be a bit controversial, but I think you can have too much oil. If the bed is sticky it will hold swarf, dust, sanding/grinding particles etc. The last thing you want to do is mash a mixture of abrasive crap into your bed.

My Myford (owned for over 50  years ) gets oiled when I feel it needs it, so it is never dry. It always has a film of oil and to try and keep the bed clean it gets wiped often. I use ISO 32 hydraulic oil also known as SAE 10, in part because I use it in 25 L drums. Whilst oiling is important preventing the particles/swarf getting into the works by wiping is better, all the more so with cast iron or small chips of hard materials eg stainless.  Noel.