Is there a period after which the question can not be deleted? Similarly for upvoted questions and answers. Maybe for deleted questions assign a unique hash code. Thus we can reread in the future (usually for the author and for those who marked as a favorite (and/or upvoted)).
No, questions can be deleted at any time, there is no period of time whereby a question is locked.
Questions can be deleted for a number of reasons:
- The poster no longer wishes the question to remain
- The scope of the website changes somewhat and what was considered a suitable question is no longer appropriate
- The question could be identified as being plagirised from elsewhere so the original owner requests it be removed...
Unfortunately there is no way to ensure they question is always available I'm afraid. Older ones are less likely to be deleted than newer ones, particularly if they have been upvoted a fair number of times so that may give you a hint as to the future of those questions, but anything can happen in the future.