These kind of follow up contracts fall into several different categories:
Maintenance (something done on a scheduled basis)
Updates (something scheduled, but done as the updates occur)
Service (which usually means be there within a set time frame if something goes horribly wrong)
Each as a reasonable price scale... it will be up to you and your client as to what that range will be.
In the first category I consider my time for the work and set a flat price per month.
Second category I charge by the hour, scheduling as soon as possible after updates are made available.
Third category the rate goes up tremendously since I have to drop whatever I am doing to respond within 4 hours.