Drop out rates depend on a lot of factors (as I'm sure you're aware); to name a few: 

How far apart are the different parts of the study?
How generous are the rewards?
How long do the different parts take?

We've had studies with 0% dropout rate and as high as 50%. A typical study would be somewhere in between these extremes. 

Here are examples of big, successful longitudinal studies run via Prolific:

https://www.prolific.ac/studies/572b39abf1d1a50009d14918
https://www.prolific.ac/studies/58208bd09b80f50001ba1d82
https://www.prolific.ac/studies/5759911d33577a00060af20d

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