Depending on the type and size of the sign and your surrounding environment, maintenance and cleaning schedules will vary. In the suburbs signs will require less attention compared to those in industrialized areas because it is just less likely to be damaged.
Just because the outside may seem clean does not mean that the inside is in the same condition. Have someone clean the inside of your electrical signage. If you open up the body of your sign you may find things such as dirt, debris and pests living inside that cause damage. It is always a good idea to have regular inspection even if it is just to change out any burned out lights.
Having shade protect your sign from elements can vastly prolong the life of your signs. Protecting your signs from the sun or rain will extend the life of your signs making it well worth the investment.
Always have an eye on the condition of your signs and contact your signage company as soon as you see damage to halt any further degradation. Prevention is a lot more cost effective than replacement.