Our basic billing block for a visit is 30 minutes.  With the usual routines of getting dogs in and out of houses and apartments, you can generally count on a walk of 20-25 minutes.  Sometimes longer, depending on routing. 

When the overall high for a day breaks 80F, we shorten walks a bit to spare the dogs, and limit our personal exposure to excessive heat and poor air quality.  When it breaks 90F, we shorten the walks considerably, for the same reasons.  If you've lived through a District summer, you likely know why.