Odds of Hanging On
Realistically, you'll have two factors to consider. First, you'll need the physical stamina and mental temperament to actually do the job. If you're a tenured professor with a nice cushy university office and a grad assistant or two, you're basically set for the long term.
But let's say your job requires you to bust it every day, digging and chipping for fossils for hours on end. You also get your share of weather extremes, biting and stinging insects, and complaints from ticked-off students who had no idea field work actually involved working in a field. Here, your odds of hanging on depend on how long your body and sanity hold up.
You've also got one more variable to juggle: funding, or lack of it. Maybe you work for a research institute, a museum, or a university, and approval for your department's budget always goes down to the wire. You might want to keep your ear to the ground, make sure your payroll checks clear, and always have an alternative plan if your funding bottoms out.