We took the swing away at about five months I think - as soon as he started trying to sit up/roll out of it. Ours was rated up to 25lbs and he still fit in it very well, but once they start squirming around it's dangerous to use so we took it away.
What we started doing once the swing was gone is putting him in his exersaucer or doorway bouncer, or we'd sit him on the floor with pillows beside and behind him, or put him in his Bumbo with toys in front of him.
Now that he can roll & creep, the only reliable way to keep him stationary and occupied is with the doorway bouncer or exersaucer. The more mobile they get, the tougher it is to take your eyes off them!