For anything Catholic, and in general, I always recommend the Douay-Rheims bible. It's both less archaic and more accurate than the KJV. It also, of course, has the Deutero-Canonicals etc.
But even other bibles translate this differently from your bible (not having "until an opportune time").
"And all the temptation being ended, the devil departed from him for a time"
"And the devil, having completed every temptation, departed from him for a time."
Even a more liberal/modern translation, like the ISV:
"After the devil had finished tempting Jesus in every possible way, he left him until another time.
Other translations differ still, having it a "season" for which the Devil ceased tempting Our Lord:
"And when the devil had completed every temptation, he departed from him for a season."
All of this meaning the Devil didn't wait until an opportune or specific time per se, as if he was waiting for Christ to sit down in order to tempt Him or something, he is alays tempting everyone, most of the time. And Christ had a human nature, and so was vunerable -but nonetheless resistant to- any temptations the Devil could offer.