Short answer: It is mandatory to set one on individual proposals for a maximum of two years from the proposal date. Be sure to build in sufficient buffer times (1-6+ weeks).
Having a guaranteed delivery date helps clients feel more confident about the commission. It protects them in case of possible artist scams and enables you to schedule work appropriately based on priority.
Build in 1-6+ weeks of buffer time!
We recommend always building in appropriate buffer time. Depending on the type of work you're doing, and how well you can estimate your workload, you may consider adding 1-6+ weeks.
Ex. if you're starting on it today and you anticipate taking 3 days, add 1 week for flex. If you're starting in 2 months, add 6 weeks just in case other things in your queue get delayed / whatever other reason.
Clients will always be happier having their expectations exceeded than delayed.