That's if he fights the top right of the bat! many P. fought a different discipline before he started boxing. If Mcgregor takes his time and doesn't start out wanting to fight the best in the top 10 or even top 20 he could possibly get a title in a few years.
UFC is a pretty shallow pool let's remember. You look at welterweight in boxing and there's over 2000 guys active within the year. UFC has less than a 100 guys at lightweight, 155lbs. Even if you add up the other lesser orgs you're not looking at more than 150-200 dudes.
SOme MMA fighters have beaten K1 fighters in K1 rules in the past Conor will do better than many would think He won't immediately be elite elite level but there are a hell of a lot of good boxers he could beat
Conor could possibly quit in his first match He quit rather easily against Diaz the first sign of trouble
Stamina could be problem as i said he should start from botom. 5 years to bild him up. Doubt that will happen as he is after money
Only thing about Conor that would surprise me is if he acted like a gentleman once in a while instead of a loudmouthed boor.