Assume you are fishing stillwaters ?
In theory, there should be an infinite variety of retrieve speeds depending on what you are fishing and trying to achieve with it.
In practice, suggest you start off by trying to master these;
- Let the fly drift around at the same speed as the breeze takes the floating fly line and just keep in touch with it
- A steady figure of 8 retrieve - you can vary this from slow to medium speed
- Quick stripping through the rod rings
- A fast constant speed retrieve, variously known as "roly poly", "milking the cow" and other expressions that degenerate to the downright rude

The first two are - generally - for imitative or suggestive patterns and the second two for lure or attractor patterns.
Keep it slow and steady in colder water temperatures in late autumn, winter and early spring; the quicker retrieves are for warmer summer temperatures.
All of the above comments have exceptions to them