If you follow the 'five minute rule' then a 6 month old can have 30 mins exercise per day, you can split this into 2 if you like(ie 2 x 15 mins walks)
We have a 6 month old and you do have to be careful as they will go for much longer but we are restricting his exercise until he's at least 12 months. Socialisation is still much more important than actually exercise, as it's an on-going thing, so we make sure he has at least one 'socialisation' outing per week ie taking him somewhere different, such as a fair, market etc, or, in our case, different shows.
HTH
Emily :)