I believe - and enforce - 180 days from the date the permit was issued. I'm not keeping track of the actual day you begin construction (and starting the clock on the day of your first/last inspection makes no sense at all) - for the purposes of the code, your construction starts/permit is valid when I sign and date your permit. If you can't get it built in 6 months, then you owe me a request for an extension.
On day 179, if you come in and sign your name to a paper asking for an extension, you get 90 more days (code says, "not more than" 180, our policy is 90 at a time). And I'll do that on day 89 of as many 90 day periods as you want, as long as some kind of progress is being made.
If you come in on day 181, you're paying me for a new permit and your 180 starts over from that day.
If I don't see you at all after day 180, then the Property Maintenance code kicks in and you've got a "dangerous" or "unlawful" structure. If you're half done building and you run out of money, then you better come up with some quick - because the first time a neighbor complains, you're getting a Notice to tear whatever-it-is down.
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How this really works is: I call you on day 179 or thereabouts and say, "Hey, shoot me an email and ask for a permit extension", then you say "ok", and send me one. I extend it for 90 and we all live happily ever after. Seems to work out for the most part..