I know my puppy can have 15 min walks at the min as she's 3 months old so I take it it's 20 mins when she's 4 months.  How long do I do ths for ( in age that is )

Until they are adult. 

Half hour at 6 months an hour at 12 months and so on.

To be honest I think few people will do more than 1.5 hour walks regularly (at which point they would be 18 months).

Some very active small breeds who mature a lot sooner may be able to have as much as you want by 18 months.