Almost instantly, but not always in the same second. Your purchase is credited to your MetaFlight account right after payment, then the MetaFlight app picks it up on its next sync. That usually takes under a minute and can occasionally run to a few minutes. If you do not see your FlightCoins or your subscription straight away, there is nothing wrong — close and reopen the app, or wait for the next sync. Contact support only if it has still not appeared after about ten minutes.