In a polluted, unsafe city? To the taxi stand.
In a very nice place? No limit.

I've once walked from Amsterdam to Breda because I wanted to experience the Dutch countryside. I've also walked the West Highland Way in Scotland, one of my favourite place in the world. I walked from British Columbia to Oliver in Washington state once because I wanted to try a restaurant there. The CBP agent I talked to at the border couldn't believe I was walking there, he even offered me to wait for the end of his shift so that he could give me a ride.