If you have the bike over the included rental time (60 minutes), you will get a warning. The next time you deliver it too late, your subscription will be suspended for the rest of the season.
If you want to be able to use the bike for a longer period of time, you can check off for extended rental in the app and gain access to the same bike for 6 hours and 60 minutes in total. You can read more about how that works here.
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