I would say about a 30 min would be ok.
Although I would consider a few things. Are your guests local? if lots are travelling and dont know the area, travelling between venues and getting lost may be a headache. COuld you bus people between venues? Lots of elderly people may find it difficult having to travel, again a bus would fix this. Also think about the roads between ceremony and reception e.g. M25 may be a problem, or city centre at a rush hour/busy time. But a more rural area may be easier.
Were having all of ours in a city centre, with reception a 2 min walk from the church so all guests can be 'hearded' round the corner without losing any!!