Walking in warmer weather

My basset is used to going for 30 min walks everyday, but now the days of 70-75 degree weather are pretty much gone now that summer is starting. I know he still needs his exercise, but the temperatures where I live are getting to be 85-90 degrees (today was 88). How long is a safe amount for him to walk outside when it’s hot? I already know the pavement rule, and if the pavement is too hot, he’s staying inside. I’m really just talking about the heat alone.