When’s the last time we saw a movie?
Yes, that's correct. "When's" always means "when is" - it will never mean "when was". I don't know why that is, but that's just how the language is!
Do you say "when's the last time you went hiking" or "when was the last time you went hiking"?
Both. As an American "when was" and "when is" and "when's" are used interchangeably. "When was..." sounds slightly more correct and formal and "when's" sounds rather casual, but I would say both in the same conversation and the meaning would be the same.
As Heather mentioned, we will use both. To me, either one sounds correct honestly. There are also times where we will say "When's" in a situation where "when was" is actually the correct grammar, but "when's" is often used casually because it is easier to pronounce than "when was".
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