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. 

There are some steps you can take to minimise the attrition rate. You should clearly communicate your study information (i.e. expectations of participants, reward structure, time gap between the phases of your study). You could also screen for those with at least 10 previous submissions and a 95% approval rate on Prolific, to ensure you are obtaining active and committed participants.

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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