Hmmm ... let's see (these are all estimates because I've never kept track of mileage per bicycle)

20 year old Venture - 16,000 miles
20 year old Maruishi - 15,000 miles
30 year old Gitane tandem - no idea
5 year old Mongoose mtn bike - 12,000 miles
4 year old Giant - 6000 miles
2 year old Marinoni - 12,000 miles