One of the old-timers will be able to tell you more about the frequency of new plans, but my impression was that it's every couple of years. They only get better, though, never worse.
You don't really need to worry too much about signing up and then having a new better plan appear, either. Dreamhost has always allowed people to "upgrade" to the new plan. The "remainder" of your prepayment is applied to your new plan. For example, if you signed up for a one year $119.40 plan and switched after six months, $59.70 would be applied to your new plan. (BTW, this applies even if you used a $97 promo code to get your $119.40 one-year plan for only $22.40).
Hope this helps!
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