I think the standard is that after 18 years of disease activity, you should have a scope every 2-3 years to check for dysplasia and colon cancer, especially with colonoic inflammation. Before that, most GIs like to scope Crohns patients about
every 5 years to track damage and disease activity, but they also do them "as needed" when there are signs of strictures, fistula, excessive bleeding or a whole range of situations. Like the disease itself, every one's scope needs are different. Overall, the more serous and active the disease, the more frequent the need for scopes. Conversely, the more stable the disease, the less often they are usually suggested. Finally, if you have a history of polyps or first degree relatives with colon cancer you should probably be on a 2-3 year interval anyway, separate and aside from any CD needs.