
The advice is that the puppy has 5 minutes of formal walking per month of life so a 3 month pup 15 minutes up to twice a day.
It is easy enough to walk a puppy five minutes from home or car, and then sit and watch the world go by, so can be out much longer meet people see traffic etc.
At 4 months 20 minutes 6 months 30 minutes, 9 months 45 minutes and hour at a year and so on.
Also of course pup can be taken out in the car, or if you don't drive on the bus etc (did that a lot for the first 15 years of dog ownership).