Don't forget, Land Before Time is meant for little children, so of course they are "babied up".
And just from looking at that, you'd know that didn't take five minutes to draw. For instance, such an angle is hard. It isn't like a profile or sideview. Such an angle takes practice, and takes more time than a profile or sideview, if I remember correctly.
Also the coloring itself would have taken many hours, perhaps even days, to do.
If I had to judge, I'd say that cover was sketched, inked, and colored in..altogether 3 days or so. But maybe I'm just exaggerating.
But yeah I agree with Threehorn. It's impossible to create a cover like that, with such a difficult angle, in only 5 minutes.