Structure
A retainer buys a defined block of production time per period, with a written list of what it can be spent on and a review point.
Rollover
Unused time does not accumulate indefinitely. A short rollover window is normal; unlimited rollover turns into a debt neither side can schedule.
When a retainer is wrong
If your needs are lumpy — nothing for two months, then a large project — booking projects individually is cheaper and more honest.
Ending one
Notice period is stated up front. Ending a retainer does not affect any project already in production.