Usually, 1-3 days will pass before getting an answer but it can also be minimum two hours before the shift starts! If you do not have time to wait for an answer, you can always draw back your application without it counting as a cancellation, as long as your application hasn't been confirmed yet.
Furthermore, when applying for many shifts in the same period, your applications will be cancelled automatically, if you're confirmed for a shift👍🏼
Applications will also expire at one time. On regular jobs, the applications will expire 6 hours before the beginning of a shift and at acute postings, 30 minutes before (a job posted within 24 hours before the start of the shift). In other words, if an application expires, it is the same as if you hadn't gotten the freelance job.