No zman, as stated deending on market it can actually have a 1 minute AP rule. It is all about market, competition and season. I know a certain European market where you can get an I fare as a walkup in the Winter and Fall, but a huge AP for the Spring and winter. Hell a few years ago not only were I fares from ISt available until the lastminute, but one could get a companion I fare for 50% off. Why? because THy was ofering that deal and as DL was already at a competitive disadvantage that winter due to lowering frequencies to 4x, they had to match the deal. The key isn't so much the flexibility or lack of it, but generally as posted above the way that they are effectively non-reroutable. I recall once flying on an I fare from Nice to JFK. I then found that I had to also go to LAX. Already knowing the fare would be ridiculous I called and asked how much it would be to add LAX onto my ticket (Had I done it before issuance it would have been $300-$400),the answer after a long hold was $4200. Obviously a seperate ticket for about $500 was a better deal.