I would expect that most office-based Home Counties-London commuters will most likely do 2 days a week or thereabouts in the office and 3 from home.
Of course there are plenty of jobs that don't suit home-working, but pretty much all office jobs[1] do in some form.
At a rough guess, I'd say we will end up with Friday levels of commuting all week on average.
[1] I don't mean offices that have customers e.g. tax offices, I mean administrative type office work.
I'm not sure that all see much of a drop off in rail passenger numbers, as whilst there'll be a reduction in the number of days current rail users use the network a reduction in the number of days people work can shift the balance from car ownership (or at least a reduction in 2+ car households).
As the big costs of cars (purchase, instance, VED, servicing, etc.) don't alter overly much with large drops in milage.
Due to the way we travel a 2.5% shift from road to rail would offset a 20% drop in rail use.