My dog is about the same size, and we walk twice or three times a day for 20-45 minutes each. When it's super hot, I try to limit the longer walks for early in the morning or dusk when it's not quite as bad. We also try to stick to shady or wooded areas. We typically walk along a small creek. When it's really hot, we end up
in the creek.
Since my dog is brachycephalic (smushy-faced), extreme heat is more of a concern than with normal nosed dogs. I have a weather-bug thing installed on my computer, so I can check the temp and heat index before we leave. If the heat index is over 75F, I keep the walks short.
But mostly, I just keep an eye on him. If he's panting excessively and lagging behind, we'll cut it short. If not, we take another lap around the park.