There is no specific rule but you need to be 90 mins away if doing a home standby.
Lots of crew use public transport but a lot drive cars. Public transport is really only a problem if you have a really early report when you might need to use a night bus if living in London or a taxi if elsewhere.
I know a lot of Mixed Fleet crew live on the South coast and other places further north of the country but because of the Mixed Fleet flying mix of shorthaul and longhaul, some are finding it hard and having to stay with friends or get a hotel sometimes.
I am sure you could cope fine if you lived about 2 hours away but any further and you might find it hard to commute because under Mixed Fleet rostering you sometimes only trigger one day off after a trip or you could have a run of shorthaul there and backs in a row.
Having said all that, I know that some of them are trying to commute but not sure how well they are getting on with it.
Hope that helps.