What are the rules/guidelines for bumping old questions? (old as in 1 year or more) What is usually the maximum time before a question is considered too old to bump?
Bumping of a Question, old or not, does not of itself signify anything constructive, re the mission of Math.SE. Bumping occurs as a side-effect of editing or answering a Question and serves the purpose of bringing these changes to the Community's attention.
If an Edit or Answer is worthwhile, even for an older Question, then the bumping that occurs is generally unobjectionable, certainly a secondary concern at best. Thus the age of the affected Question is at best a tertiary concern.
The Community process will occasionally bump random questions without any activity triggering it. That tells me that bumping old questions is indeed desirable, as long as it doesn't affect too many in too short a time. And I'd rather see a post bumped because it got improved than just bumped with no improvement.
So if you want to answer a question or improve a post, I'd say go ahead, improve our site with valuable content. If the questions you process are similar in nature (e.g. because you're going over a given tag listing, or all posts of a given author, or deliberately searching for some common misspelling or dead links or something like that), make sure to limit your bumps so that they don't come to dominate the front page. Perhaps check there is only one or two of them in the list at any given time?
If, on the other hand, you don't intend to substantially improve the post, but only edit it for the sake of bumping the question, then I'd rather not see that tool being abused. If you want to draw attention, offer a bounty. Or hope for luck and wait for the Community process to bump it.