If I have conquered the entire continent do I get those bonus continent troops on each turn I play or just one turn after I have conquered it?
You receive bonus troops for a continent each time you begin your turn in control of every district/nation on that continent, for as long as that remains true. That is, if you lose control of any district on the continent, you will not collect the bonus troops at the start of your next turn.
from Rules of Risk
In addition, at the beginning of your turn, you will receive armies for each continent you control. (To control a continent, you must occupy all its territories at the start of your turn).