My daughter leans forward in hers, causing it to stop swinging. Also, she's so tall that I'm afraid her soft spot's going to press against the bar that attaches the mobile to the swing. She's 5 months old.
I stopped about 4 months old because my daughters legs didn't fit underneath the table properly and she loves to kick! Unfortunately I bought one with the table that doesn't come off!! I would say anytime I would stop using it. Good luck.