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.
Offering bonus incentives to participants who complete all parts of your longitudinal study is also a good way to minimise attrition - you can read about how to do this here: http://help.prolific.ac/top-ups-payments-refunds/can-i-make-bonus-payments