I didn't know I should have been weaning mine off it for a few months, so I didn't even try until my pediatrician told me to at his 4 month checkup. At that point it took me several weeks. I don't remember exactly how long.
What I ended up doing that worked was start him out with it just like usual, and then partway through the feeding when the flow was good and he looked like he was getting good suction and latch, take him off and remove it and put him back on quickly. I'd just slip it off and put him right back. It took a few tries (or a few days) to get him to even go back on once I'd taken it off. When he couldn't re-latch, I'd put it back and try again on the other side. Once he was able to latch back on himself I did that for a few days. Then I started trying to get him to latch on from the start. That took a while too because he would get impatient for the letdown, which wasn't as hard with it on. Finally he was able to latch on and start by himself. I still kept it close for a couple weeks because occasionally he would refuse and get frustrated still, so I'd put it on and then slip it off partway through.
It might take a while longer before he can do it. Our nursing was pretty well established and he was a pretty efficient nurser before I tried it. Plus I had a pretty fast letdown. If he won't do it right away, just try again in a week or so, and keep working at it.
You can also ask a lactation consultant what method they recommend to get rid of it. The pediatrician told me I should really try to wean him off it because sometimes it can cause milk supply issues. You might never have that problem, but it is probably a good idea to try anyways, because nursing while we were out was an ordeal making sure I always had one with me, plus spares. The moment of panic I experienced when I had a hungry baby and no shield (I thought) was enough to make me try it. Good luck!