It depends on how you choose to send the funds to your FlashFX account.
If you deposit by a regular bank transfer it could take up to 24 hours for it to arrive in your FlashFX account. Banks only process funds Monday-Friday, and not on holidays, so if you deposit on a weekend or after a certain time of day the funds will take longer to arrive.
If you deposit via NPP the funds arrive in real-time. FlashFX processes NPP deposits 24/7.