If you join either infantry or armour, you will get posted to a unit after your training. It may be the unit of your choice, or it may be where the Army needs you most. So, here's the formula:
A. If you start now, and estimate a year for recruiting and six months for training
B. And you hope that the planned cycle of unit rotations overseas doesn't change, and pick a trade based on that
C. And get posted to your unit of choice.
D. And land in one of the companies/squadrons designated for a tour.
E. And the government doesn't change its mind on the mission.
F. And the Army doesn't change its mind on Task Force structure.
G. And you "DAG Green" all the way through the pre-deployment training cycle.
H. And you don't piss off your chain of command and get dumped to the rear party for being a dink, or some other technical term for administrative or disciplinary burden.
I. And there's not another election that pulls us out of Afghanistan (or wherever) at the last moment.
J. And you don't get appendicitis two days before your departure.
K. And the sun doesn't go nova.
Then:-
You might get an operational tour within a three-year basic engagement.