what is imac life span?

Until it breaks. 🙂

However many users get 6-10 years out of theirs. Everyone uses their iMac differently, so really your question is really unanswerable. My first iMac had a HD fail after 8 years, however newer computers come with SSDs so in theory they should last much longer than a HD.

Well a 2007, is a old computer. Perhaps it is time to consider replacing it.

No idea as there are too many variables in how it is used, etc. Electronic parts can die at any time; some hard drives last for 5 - 8 years, others die within a year. So, it is impossible to predict. My personal expectation is that it lasts for at least 3 years (while it is covered by the extended Applecare); hopefully, anything major will come up during that time and be replaced for free. I've never had one for more than 3 years, so I don't know what my expectations would be at that time. This is partially because any computer is outdated within 6 months or so, so it's advisable to update and sell your old one while it is still covered by a warranty (which gives the buyer an assurance that he can get it fixed for free).