Four times a year, with up to two seconds added each time should put an upper bound for adjustments accumulated over 500 years to:
500 years * 4 updates/year * 2 seconds/update = 4000 seconds = 66 2/3 minutes
So an hour in 500 years seems to require adjusting almost as much as we can, which isn't realistic.
The original proposal allows updates four times a year, though only the July and December updates have been used.