I would tell my patients it is safe to fly in 4 weeks after a "Mommy Makeover," although ideally, I would like my patients to remain local for 6-8 weeks postop if they can, as I like to follow their progress closely in the first two months. Most wound healing (to the point that the incisions won't reopen) occurs in the first 3-4 weeks, and the risk of blood clots should be minimal for a 2-hour plane flight 4 weeks after surgery. However, if you run into difficulty and are somewhere where you do not have access to a board-certified plastic surgeon, you might have issues. I would also advise any patient of mine to "take it easy" on exercise and exertion during their absence.
Having said that, it is pretty common that patients want to take a mini-vacation and enjoy the benefits of their new body image within a month or two of surgery; after all, isn't feeling better the whole point of cosmetic surgery?