To the OP, I wouldn't compare myself to the responses here. I'm going to guess that some people are either lying, or they're not playing the songs correctly or well. There are some very unrealistic claims being made for a first song that someone learned. As for me, the first song I learned on bass was Teentown. Took me about 15 minutes. Maybe 20. Also, to the OP, while you're working on Welcome to the Jungle, it might serve you well to work on some other easier material. Maybe do a search for simplest basslines. Songs like Blitzkrieg Bop, Money, With or Without You, 7 Nation Army, etc. It will help train your ear, help you learn to learn, and also build your confidence. Welcome to the Jungle, if played correctly, is not one of the easier songs to learn on bass.