That answer depends greatly on your definition of 'good'. If by good you mean learn how to prep your metals, attach a ground strap within a foot of your welding area, and get good penetration with a MIG so you can get a job done, then maybe not too long. Definitely less if you are used to soldering as it is very much the same process just scaled up a bit. If you want to be good enough to make the pretty welds we all love to look at on social media this can take a while and requires lots of practice like playing a band instrument. And if you want all of your welds to pass X-ray tests then you should have a bunch of experience laying the pretty welds. One of my first jobs in the automotive industry was at a Midas muffler shop 20 years ago, now. There I learned to weld and also to use a pipe bender. And today I can make stuff like this: https://www.digitalcorvettes.com/thr.../#post-3085545 . And this: https://www.digitalcorvettes.com/thr.../#post-3441096 . And also this: https://www.superchargerforums.com/t...ring-gto.3135/ . Or even this: https://www.superchargerforums.com/t...n-a-7-0l.3130/ .