There is no specific guidance on that. The only guidance is from what I referenced in the first portion of my answer which is the one year mark. There is some gray area here. My advise would be that these costs aren't deductible, but you are trying to make the case that they are. So as long as you can make the case to the IRS that the expenses were deductible based on temporary location then you should be good.
Determining temporary or indefinite. You must determine whether your assignment is temporary or indefinite when you start work. If you expect an assignment or job to last for 1 year or less, it is temporary unless there are facts and circumstances that indicate otherwise. An assignment or job that is initially temporary may become indefinite due to changed circumstances. A series of assignments to the same location, all for short periods but that together cover a long period, may be considered an indefinite assignment.
The following examples illustrate whether an assignment or job is temporary or indefinite.
You are a construction worker. You live and regularly work in Los Angeles. You are a member of a trade union in Los Angeles that helps you get work in the Los Angeles area. Your tax home is Los Angeles. Because of a shortage of work, you took a job on a construction project in Fresno. Your job was scheduled to end in 8 months. The job actually lasted 10 months.
You realistically expected the job in Fresno to last 8 months. The job actually did last less than 1 year. The job is temporary and your tax home is still in Los Angeles.
The facts are the same as in Example 1, except that you realistically expected the work in Fresno to last 18 months. The job actually was completed in 10 months.
Your job in Fresno is indefinite because you realistically expected the work to last longer than 1 year, even though it actually lasted less than 1 year. You cannot deduct any travel expenses you had in Fresno because Fresno became your tax home.
The facts are the same as in Example 1, except that you realistically expected the work in Fresno to last 9 months. After 8 months, however, you were asked to remain for 7 more months (for a total actual stay of 15 months).
Initially, you realistically expected the job in Fresno to last for only 9 months. However, due to changed circumstances occurring after 8 months, it was no longer realistic for you to expect that the job in Fresno would last for 1 year or less. You can only deduct your travel expenses for the first 8 months. You cannot deduct any travel expenses you had after that time because Fresno became your tax home when the job became indefinite.