They expire based on the stash calendar, so 7 days. Do NOT use the in-game mail calendar. I did this last year and they expired. I had like 3k units worth of items I was saving and lost them. Luckily, I did submit a ticket and the Kabam support team was kind enough to get me the items.
Good question. General rule of thumb? The stash screen is the one to trust when there is a contradiction.